Synthesis of Spectinomycin Analogs - American Chemical Society

7 Synthesis of Spectinomycin Analogs R. C. THOMAS, D. R. WHITE, V. H . WILEY, and D. A. FORSTER

Downloaded by UNIV OF ARIZONA on June 8, 2013 | http://pubs.acs.org Publication Date: August 4, 1980 | doi: 10.1021/bk-1980-0125.ch007

The Upjohn Company, Kalamazoo, MI 49001

Spectinomycin (1) (Figure 1) is an aminocyclitol antibiotic of moderate potency which is useful in the treatment of gonorrhoeae, especially for penicillin resistant strains. It has antibacterial activity against gram-negative and gram-positive bacteria, and lacks the oto- and nephrotoxicity problems usually associated with the aminoglycoside family of antibiotics. This broad spectrum activity and lack of toxicity make spectinomycin an attractive candidate for structural modifications aimed at enhancing its potency. The sensitivity of spectinomycin to both acids and bases has severely hampered past synthetic efforts in this area. Recently, a number of reports have appeared in the literature describing chemical modifications of the aminocyclitol portion of the antibiotic (2-7). These structural modifications have been inspired by aminoglycoside structure-activity relationships and by a knowledge of spectinomycin inactivation by R-factor mediated adenylation of the C-6 hydroxyl group. A l l changes in the actinamine portion of the molecule have resulted in the loss of antibiotic activity. The only m o d i f i c a t i o n of the sugar p o r t i o n of spectinomycin, to be reported to date, i s the s e q u e n t i a l r e d u c t i o n of the adiketone system (Figure 2). The dihydrospectinomycins ( 1 ) , prepared during the course of the s t r u c t u r e determination s t u d i e s , have diminished b i o l o g i c a l a c t i v i t y , but remain, a f t e r 16 y e a r s , the only a c t i v e analogs o f spectinomycin appearing i n the l i t e r a t u r e . F u r t h e r r e d u c t i o n a f f o r d s the tetrahydrospectinomycins (8), which are devoid of a n t i b i o t i c a c t i v i t y . In the accompanying manuscript of Dr. David R. White, t h e f e a t u r e s of the s y n t h e t i c approach to spectinomycin analogs which has been developed at The Upjohn Company were o u t l i n e d . The f i r s t s y n t h e s i s o f spectinomycin was reported. In t h i s companion paper the f l e x i b i l i t y of t h i s s y n t h e t i c scheme w i l l be shown by d e t a i l ing the s y n t h e s i s o f some analogs and the i n t e r e s t i n g s t r u c t u r a l

0-8412-0554-X/80/47-125-121$05.00/0 © 1980 American Chemical Society

In Aminocyclitol Antibiotics; Rinehart, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


122

AMINOCYCLITOL ANTIBIOTICS

ACTINOSPECTINOIC

ACID

Figure 1.

Figure 2.


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Spectinomycin Analogs

consequences a r i s i n g from m o d i f i c a t i o n s i n the sugar fragment w i l l be described.


Discussion The f i r s t key steps o f t h e s y n t h e t i c sequence a r e shown i n schematic form i n F i g u r e 3. The f l e x i b l e nature of t h i s approach i s immediately apparent i n that the a m i n o c y c l i t o l and sugar components may be v a r i e d a t w i l l and s t r u c t u r a l m o d i f i c a t i o n s i n e i t h e r can be made before o r a f t e r the coupling o f these p i e c e s . Ν,Ν'-dicarbobenzyloxyactinamine i s shown as a r e p r e s e n t a t i v e p r o t e c t e d a m i n o c y c l i t o l i n t h i s i n s t a n c e . This paper w i l l be l i m i t e d t o t h i s example, w h i l e focusing on m o d i f i c a t i o n of t h e sugar component. A c e n t r a l f e a t u r e o f t h i s scheme i s the use o f the Lemieux c o u p l i n g process (9) f o r formation o f the g l y c o s i d e l i n k a g e . This process provides adequate s e l e c t i v i t y f o r the actinamine C-5 hydroxyl group and introduces the r e q u i s i t e C-2 carbonyl group as i t s oxime. Subsequent deoximation a f f o r d s the d e s i r e d t r i c y c l i c hemiketal system which can be subjected t o f u r t h e r m o d i f i c a t i o n s and d e p r o t e c t i o n . In the p r e v i o u s l y d e s c r i b e d spectinomycin s y n t h e s i s , Lglucose was employed as the sugar component due t o the preference f o r α-glycosylation i n t h e Lemieux c o u p l i n g r e a c t i o n . The a-L anomer corresponds t o n a t u r a l spectinomycin anomeric s t e r e o chemistry. Subsequent transformations i n v e r t e d the stereochemis t r y a t carbon - 5 t o that o f t h e n a t u r a l product. The consequen ces o f employing the antipode D-glucose i n t h i s s y n t h e t i c scheme are shown i n F i g u r e 4. Reaction o f t h e D-glucose derived g l y c o s y l a t i n g reagent w i t h the actinamine d e r i v a t i v e provides a mixture o f symmetrical adducts, w i t h t h e α-anomer predominating as a n t i c i p a t e d . The r a t i o o f α t o $ anomers being approximately 20 t o 1 i n t h i s i n s t a n c e . N o t i c e that the 3-anomer has n a t u r a l spectinomycin stereochemistry a t both the anomeric carbon C-1 and a t C-5 . The α-anomer, which i s t h e m i r r o r image o f the intermediate employed i n the spectinomycin s y n t h e s i s , has unnatural anomeric stereochemistry. I n the case o f the α-anomer, subsequent d e o x i mation, i n t r o d u c t i o n of the C-3 carbonyl by $-éliminâtion, and enone reducton w i t h concomitant d e p r o t e c t i o n o f t h e n i t r o g e n atoms provides the enantiomer o f spectinomycin (Figure 5 ) . T h i s compound has p h y s i c a l c h a r a c t e r i s t i c s i d e n t i c a l t o those o f the n a t u r a l isomer except f o r the o p t i c a l r o t a t i o n . I t was found t o be devoid of a n t i b a c t e r i a l a c t i v i t y even when t e s t e d a t unusually high c o n c e n t r a t i o n s , thus e s t a b l i s h i n g two very important p o i n t s . F i r s t , the b i o l o g i c a l a c t i v i t y r e s i d e s only i n the n a t u r a l antipode i n t h i s s e r i e s o f compounds and second, that the s y n t h e s i s i s i n f a c t s t e r e o s p e c i f i c . I f any r a c e m i z a t i o n o f t h e anomeric center had occurred a t any p o i n t i n the s y n t h e s i s , the r e s u l t i n g n a t u r a l anomer would have y i e l d e d b i o a c t i v e m a t e r i a l . I t should be r e c a l l e d that a l l o f the ?

f

T

1


T



124

Figure 3.

CBz

OC - ANOMER

20= I

QH

β - ANOMER

Figure 4.


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asymmetric centers of the sugar except a t the anomeric p o s i t i o n are removed during the course of the s y n t h e s i s and that the anomeric stereochemistry determines the c o n f i g u r a t i o n of a l l nine asymmetric centers i n the f i n a l product. As i s shown i n F i g u r e 6, the minor oxime, the β-anomer, was s e q u e n t i a l l y deoximated, the acetates were methanolyzed and the p r o t e c t i n g groups removed by c a t a l y t i c hydrogénation to a f f o r d 4*,6 -dihydroxydihydrospectinomycin. This t o t a l l y s y n t h e t i c spectinomycin analog was a c t i v e against a v a r i e t y o f t e s t organisms, w i t h potency somewhat l e s s than the corresponding nonhydroxylated dihydrospectinomycin which was d e r i v e d from r e d u c t i o n of spectinomycin i t s e l f . Thus f o r the f i r s t time, a b i o l o g i c a l l y a c t i v e spectinomycin analog was s y n t h e s i z e d , by f u s i n g a sugar component onto an a m i n o c y c l i t o l u n i t . The success achieved w i t h both antipodes o f glucose prompted the examination of the u t i l i t y of other sugars i n t h i s s y n t h e t i c scheme. The remaining two sugars to be discussed were examined as probes to answer chemical questions r e l a t i n g t o the problem o f hemiketal f o l d i n g that was discussed e a r l i e r i n Dr. White's manuscript. These sugars have provided i n t e r e s t i n g b i o l o g i c a l i n f o r m a t i o n as w e l l . The r e s u l t s obtained w i t h D-arabinose are shown i n F i g u r e 7. One of the more a t t r a c t i v e f e a t u r e s of t h i s pentose i s the high p r o p o r t i o n of B-D-anomer produced i n the c o u p l i n g process. Thus r e a c t i o n of the n i t r o s y l c h l o r i d e adduct of d i - O - a c e t y l - D - a r a b i n a l w i t h N,N -dicarbobenzyloxyactinamine gives the d e s i r e d 3-anomer i n 35% y i e l d , along w i t h l e s s e r amounts o f the α-anomer and products of r e a c t i o n at the unsymmetrical hydroxyl groups. Deoximation gives one hemiketal s t r u c t u r e e s t a b l i s h e d by s p e c t r a l means as having n a t u r a l spectinomycin f o l d i n g . That i s , the C-4 h y d r o x y l group forms the c i s - h e m i k e t a l l i n k a g e as opposed to the C-6 h y d r o x y l . A p p l i c a t i o n of e i t h e r the $ - e l i m i n a t i o n method o f C-3 ketone i n t r o d u c t i o n or a c e t a t e h y d r o l y s i s w i t h subsequent Nd e p r o t e c t i o n i n both cases a f f o r d s the 5'-demethyl analogs shown. Both of these compounds possess moderate b i o a c t i v i t y w i t h the keto compound being more a c t i v e as expected. A n a l y s i s o f s p e c t r a e s t a b l i s h e s only the n a t u r a l spectinomy c i n mode of hemiketal f o l d i n g f o r 5 -demethylspectinomycin (Figure 8). T h i s i s an i n t e r e s t i n g r e s u l t i n l i g h t of the f a c t o r s a f f e c t ing the mode of hemiketal f o l d i n g described i n Dr. White's manuscript. I n spectinomycin, the e q u a t o r i a l methyl group at C-5 i s one of the f a c t o r s that helps s t a b i l i z e the n a t u r a l f o l d i n g i n v o l v i n g the C-4 h y d r o x y l group, r e l a t i v e to other modes of f o l d i n g which would r e q u i r e the methyl group to adopt an a x i a l o r i e n t a t i o n . Even i n the 5 -demethyl analog, which l a c k s t h i s stereochemical b i a s , anomeric e f f e c t s and other f o r c e s are e f f e c t i v e i n maintaining n a t u r a l folding. The e f f e c t s o f a l t e r e d stereochemistry i n the sugar r i n g have been f u r t h e r s t u d i e d by e x p l o r i n g the a p p l i c a t i o n of L-rhamnose i n t h i s scheme. F i g u r e 9 d e p i c t s the question at hand. That i s ,


f

f

?

f

f

f




126

4',6'-DIHYDR0XYDIHYDR0SPECTIN0MYCIN

Figure 6.


THOMAS ET AL.



+ OC -ANOMER and UNSYM. ISOMERS I * CBz β — ANOMER

(35%)

Figure 7.




128

3 NATURAL

FOLDING

(OBSERVED)

Figure 8.

L-RHAMNOSE

H 5'-

NATURAL

SPECTINOMYCIN

FOLDING

ABNORMAL

EPISPECTINOMYCIN

FOLDING

Figure 9.


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what i s the e f f e c t of an u n n a t u r a l c o n f i g u r a t i o n a t carbon 5? We see t h a t n a t u r a l f o l d i n g i n the 5'-epimeric s e r i e s would r e q u i r e an a x i a l o r i e n t a t i o n of the methyl group, a d e s t a b i l i z i n g f a c t o r . One o f the a l t e r n a t i v e f o l d i n g modes, which i s shown i n the F i g u r e , would a l l o w an e q u a t o r i a l C-5 methyl group, but the h y d r o x y l group a t C-2 must now be i n the l e s s favored e q u a t o r i a l o r i e n t a t i o n . The anomeric e f f e c t a t carbon l i s e q u i v a l e n t i n these two s t r u c t u r e s , and n e i t h e r r e q u i r e s the presence o f boat r i n g s . The chemical steps used t o evaluate t h i s question are shown i n F i g u r e 10. Treatment o f commercially a v a i l a b l e 3,4-di-O-acetyl-L-rhamnal w i t h n i t r o s y l c h l o r i d e g i v e s the u s u a l ois adduct, i s o l a t e d as the n i t r o s o d i m e r . Reaction w i t h N,N -dicarbobenzyloxyactinamine a f f o r d s the expected α-L-anomer i n good y i e l d . Deoximation a f f o r d s a s i n g l e hemiketal which does not have n a t u r a l f o l d i n g as e s t a b l i s h e d by NMR. Examination o f s p e c t r a l data and molecular models suggests the s t r u c t u r e shown. Abnormal f o l d i n g i s expected at t h i s stage because n a t u r a l f o l d i n g would r e q u i r e the s u b s t i t u ents on carbons 3 , 4 , and 5 t o a l l adopt a x i a l o r i e n t a t i o n s . The stage i s now set f o r the β-elimination process f o r gener a t i o n o f the C-3' ketone. I n the present i n s t a n c e , the l a c k of an acetoxy group on C-6 precludes any second e l i m i n a t i o n and t h e r e f o r e precludes a l t e r a t i o n o f the u n n a t u r a l C-5 stereochemistry. S p e c t r a l s t u d i e s show t h a t the r e s u l t i n g acetoxy-ketone and the d e r i v e d hydroxy-ketone are both abnormally f o l d e d . C o n s i d e r a t i o n of molecular models suggests the s t r u c t u r e s shown, which c o r r e s pond t o the mode of abnormal f o l d i n g t h a t was d e p i c t e d i n the previous f i g u r e . Removal o f the p r o t e c t i n g groups y i e l d s a product having only very weak a n t i b i o t i c a c t i v i t y . S p e c t r a l a n a l y s i s o f t h i s product shows mainly a s i n g l e u n n a t u r a l l y f o l d e d compound, assigned the s t r u c t u r e shown, along w i t h minor i m p u r i t i e s which could be a l t e r n a t i v e f o l d e d forms o r a c y c l i c compounds. Taken together w i t h the r e s u l t s obtained i n the 5 -demethylspectinomycin case, the f o l l o w i n g trend emerges. The e q u a t o r i a l 5'-methyl group i n spectinomycin, w h i l e c e r t a i n l y s t a b i l i z i n g the n a t u r a l mode of hemiketal f o l d i n g over other p o s s i b i l i t i e s i s not a requirement. The anomeric and other e f f e c t s i n the 5'-demethyl analog are s u f f i c i e n t t o enforce n a t u r a l f o l d i n g . These e f f e c t s , however, are not enough t o overcome the b i a s of an a x i a l s u b s t i tuent a t carbon 5*. Thus, the 5 e p i analog o f spectinomycin adopts a l t e r n a t i v e hemiketal f o l d i n g . 1

1

f


f

1

1

f

1

f

f

f

Summary These r e s u l t s e s t a b l i s h the g e n e r a l i t y of the t o t a l s y n t h e t i c method f o r producing b i o l o g i c a l l y a c t i v e analogs of spectinomycin. I t provides ready access t o dihydro analogs as w e l l as p r o v i d i n g a v e r s a t i l e method f o r i n t r o d u c i n g the d e s i r a b l e C-3' c a r b o n y l group without r e l y i n g on s e l e c t i v e p r o t e c t i o n o r o x i d a t i o n . I t i s a short e f f i c i e n t sequence, by which a c t i v e analogs can be pre-




130

Figure 10.


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pared from known s t a r t i n g m a t e r i a l s i n as few as four s y n t h e t i c s t e p s . Thus, i t compares very f a v o r a b l y w i t h approaches r e l y i n g on p r o t e c t i o n and subsequent m o d i f i c a t i o n o f i n t a c t s p e c t i n o mycin p r e c u r s o r s . The s y n t h e s i s of enantiomeric spectinomycin by t h i s technique confirms the s t e r e o s p e c i f i c i t y of the s y n t h e t i c scheme and estab l i s h e s the dependence o f a c t i v i t y on n a t u r a l absolute c o n f i g u r a t i o n . The r e s u l t s obtained i n the D-arabinose and L-rhamnose d e r i v e d analogs have shed some l i g h t on the f a c t o r s a f f e c t i n g hemiketal f o l d i n g and have shown the e f f e c t s of abnormal f o l d i n g on a n t i b i o t i c a c t i v i t y .


Literature Cited 1. Wiley, P. F., Argoudelis, A. D., and Hoeksema, H., J. Amer. Chem. Soc., (1963), 85, 2652. 2. Carney, R. E. and Rosenbrook, W., J. Antibiotics, (1975), 28, 953. 3. Rosenbrook, W., Carney, R. E., Egan, R. S., Stanaszek, R. S., Cirovic, M., Nishinaga, T., Mochida, Κ., and Mori, Y., ibid., (1975), 28, 960. 4. Rosenbrook, W., Carney, R. E., Egan, R. S., Stanaszek, R. S., Cirovic, Μ., Nishinaga, T., Mochida, Κ., and Mori, Y., ibid., (1978), 31, 451. 5. Carney, R. E. and Rosenbrook W., ibid., (1977), 30, 960. 6. Foley, L., Lin, J. T. S., and Weigele, Μ., ibid., (1978), 31, 979. 7. Foley, L., Lin, J. T. S., and Weigele, Μ., ibid., (1978), 31, 985. 8. Knight, J. and Hoeksema, Η., ibid., (1975), 28, 136. 9. Lemieux, R. U., Nagabhushan, T. L., and Gunner, S. W., Can. J. Chem., (1968), 46, 405. RECEIVED

November 15, 1979.


Synthesis of Spectinomycin Analogs - American Chemical Society

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