2672
in an apparatus previously described.$ When the spectrum of 0.3 m M o-benzylbenzophenone in cyclohexane is measured several minutes after the flash excitation, i t deviates only to a small extent from the initial absorption (A,, 338 and 342 mp, E 140).1° However, 50 psec. after the flash excitation the absorption spectrum is markedly different with a new maximum a t 500 mp and a minimum a t 344 m p (Fig. I ) . By studying the optical density a t various wave lengths with kinetic spectrophotometry two transient intermediates were identified separately. A fast transient, the absorption maximum of which a t 500 mp is very similar t o the triplet-triplet absorption of benzophenone," was assigned as the n + H* triplet of obenzylbenzophenone (IT). Its rate of decay measured a t 436 mk and 380 mp follows first-order kinetics with k = 1.9 f 0 . 2 X 103sec.-I which further substantiates the triplet assignment. l 1 The slow transient, detected by the decay of optical density a t 405 mp, is tentatively identified as the enol (111). The decay is also of first 0.6 X set.-'. order with k = 9.4 The interconversion relationship of the two intermediates was established by following the variation of the optical density a t 345 mk. The optical density a t 345 mp decreases to a minimum value immediately after the flash (Fig. l), then it increases rapidly to a maximum a few msec. thereafter with a first-order rate constant, k = 3.2 >( l o 3set.-' ( *joyo),which is of the same order of magnitude as that of the triplet decay. The maximum optical density attained a t this wave length is approximately 0.1 unit higher than the final absorption. The decay of this maximum absorption to the final value also follows first-order kinetics with k = 8.7 X 1C)-2sec.-I, which is the same as the decay of the slow transient within experimental error. The above observations establish that (1) the fast transient is converted into the slow transient, i . e . ,t h e n -+ H* triplet is the precursor of the enol, and (2) the enol is not formed directly from the n --t H* singlet. If the n + H* singlet were the immediate precursor of the enol, there would be a strong absorption a t 345 mk immediately after the flash. Our observations demonstrate that photoenolization of o-benzylbenzophenone, an intramolecular photochemical reaction analogous to the type I1 process, involves a reaction triplet intermediate. Although the enolization may occur theoretically within the lifetime of the singlet excited state, i t i s actually slower than the interstate crossing of the singlet to the triplet (reaction 3). This work supports the theory that the triplet state may be a reactive state in other similar intramolecular photochemical reactions. I 2 , l 3 (9) E. F. Zwicker a n d I,, I. Grossweiner, J . P h y s . Chem., 87, 519 (1963); L. I. Grossweiner a n d E F. Zwicker, J C h e m P h y s , 34, 1411 (1961). ( I O ) After several Hash irradiations or a prolonged irradiation with a high intensity light source, t h e solution acquired a yellow t i n t with a blue f l u o r e s ~ cence. F r o m t h e reaction medium there was isolated a small yield of a dimer whose structure is being elucidated. (11) D S hfcClure a n d P. H a n s t . J . C h m . P h y s . , 2.9, 1772 (195.5). (12) P. Ausloos a n d R . E. R e b b e r t . J . A m . Chem. S O L .83, , 4897 (1981). (13) A' H. Urry and U . J T r e c k e r , % b i d .8 4 , 118 (19021. (11) T h e work a t t h e Illinois Institute of Technology is supported by t h e United S t a t e s Public Health Service, n e p a r t m e n t of H e a l t h , Education a n d Welfare (15) T h e work a t t h e University of Chicago is supported b y t h e United S t a t e s Atomic Energy Commission ( l i i ) Fellow of t h e Alfred P Sloan F o u n d a t i o n .
I)EPARTME\.T OF PEiYSICS"
ILr-rsors I SSTITLTE O F T E C H S ~ I L K Y CHICAGO 16. II.:.INOIS
VOl. 85
COMMUNICATIONS TO THE EDITOR
I,
The Synthesis of the Anomeric 7-~-Ribofuranosyladenines and the Identification of the Nucleoside from Pseudovitamin BIZ1 Sir: The final identification of the nucleoside moiety of vitamin Bl2 as l-a-~-ribofuranosyl-5,6-dimethylbenzimidazole was achieved by comparing its picrate with a synthetic specimen. Later a nucleoside was isolated from pseudovitamin BIZ and identified as 7-Dribofuranosyladenine. Although the configuration of the glycosyl linkage of this nucleoside was thought to be a,4this point has not been established by chemical means in the years since the isolation of the nucleoside. Using a recently developed methods for the synthesis of 7-glycosylpurines, we have now synthesized both 7-p-~-ribof uranosyladenine (I) and 7 -a-D-ribofuranosyladenine (11) and established the identity of the nucleoside from pseudovitamin Blz and 11.
I
"2
"2
I
I
3-Benzyladenine (III),prepared by the benzylation of adenine in N,N-dimethylacetamide in the absence of base,7 was allowed to react with benzoic anhydride to give N6-benzoyl-3-benzyladenine (IV), which was converted to its mercury derivative (V) in the usual manner.8 The mercury derivative V was coupled with tri-0acetyl-D-ribofuranosyl chloride by refluxing a xylene suspension of the two materials for 1 hr. Removal of the acyl groups from the blocked nucleoside VI was accomplished by refluxing for 30 min. a methanol solution of it containing sodium methoxide. A 51% yield
c~H~CH~ IV AcO
?Ac
Q
AcOCH~
CsHsCH2 VI
HO
OH
Q
HOCHz
c~H~CH~ VI1
(1) This work was supported by f u n d s f r o m the C. F. K e t t e r i n g F o u n d a tion a n d t h e Cancer Chemotherapy National Service C e n t e r , National Cancer Institutes, National I n s t i t u t e s of H e a l t h , C o n t r a c t S o . SA-4R-:>h1740 ( 2 ) F W Holly, C H S h u n k , E . W . Peel, J . J Cahill, J B . I a v i g n e . a n d K Folkers. J . A m . C h e m S O C . , ~ 4521 ~ , (1952) E F ZIVICKER 13) W Friedrich a n d K . Bernhauer, C h e m R e i , 89, 2.507 (19313). I. GROSSWEINER (41 X - R a y analysis o f t h e structure of t h e BII vitamins suggests the a con figuration a s well a s a t t a c h m e n t a t 5-7.5 ( 5 ) I ) . C Hodgkin, Biochem. SOC.S y m p . ( C a m b r i d g e , E n g l a n d ) , 1 3 , 28 (i9.i.i). S C Y A S G ~ ~ (footnote) (17) J A hlonl.gomery a n d H . J . T h o m a s , J . Oug. Chem.. 28, 2804 (19I;:j). : 7 ) J. W J o n e s a n d R . K . Robins, J . A m . Chem. S o t . . 84, 1914 (1962) (81 J J Fijx, S Y u n g , I . W e m p e n , a n d I . I>, l l o e r r , i b i d . , 79, 50110 (1967)
COMMUNICATIONS TO THE EDITOR
Sept. 5 , 1963
2673
TABLE I Ultraviolet spectrad---
7 -
M.P., Compounda
I
c.*
246
I1
220-222
Nucleoside from $BPZe Adenosine*
218-222
9-a-DKibofuranosyladenineh I11 IV
233
(g./lOO ml.
HaO)
-84.9 f 0 . 2 (0.351) 0 (0.397) 0' (0.262) -60.4 (0,7) 24 (0.65)
+
20 1 275-277 216
RrC
7
0 . 1 NaOH
0 . 1 HC1
[U]"D
Xmsx.
ax
Xmax,
cx
ma
10-8
mir
10 - 0
0.19
272
13.6
270
9.8
.15
273
13.3
271
9.9
273
13.6
271
9.8
.20
260
14.2
260
4.3
.17
257
.75 .89
275 300
17.5 28.2
272 231h 333
2.2 7.0 17 9
150
v'
278 11 5 31 278 14 3 28 1 VI1 a Satisfactory elemental analyses (C, H, N ) were obtained for each compound except VII, which was not analyzed * Melting c Whatpoints below 260" were determined on a Kofler Heizbank; those above 260' were determined in a capillary and are uncorrected man No. 1 paper, water-saturated butanol. Determined with a Cary Model 14 spectrophotometer. e Data from ref 3 TemElemental analyses show t h a t V is a dipurinylmercury. perature not specified. Calcd. from Radenoslne. * Data from ref. 14.
'
of crude 3-benzyl-7-P-~-ribofuranosyladenine (VII) was obtained as a brown glass. The benzyl group of this nucleoside VI1 was removed by hydrogenolysis using 5% palladium-on-charcoal catalyst in a mixture of ethanol and water a t 80' and 47 p.s.i. of hydrogen. The resulting nucleoside, obtained in 31% yield, was identified as 7-@-~-ribofuranosyladenine(I) by its ultraviolet spectrum (showing sugar attachment a t N-7) and application of the trans ruleg (indicating the configuration of the glycosyl linkage formed by this coupling must be @lo)). Coupling of the same dipurinyl mercury V with 5-0benzoyl-D-ribofuranosyl bromide 2,3-cyclic carbonate13 followed by deblocking of the resultant nucleosides with methanolic sodium methoxide gave a mixture of 3benzyl-7-B-D-ribofuranosyladenine(VII) and 3-benzyl7-a-~-ribofuranosyladenine(VIII) which could not be separated but could be debenzylated catalytically as described earlier for the p-anomer. Partition chromatography of the resulting mixture, using a cellulose powder column and butanolLwater (86:14), gave first 7-@-D-ribofuranosyladenine( I ) , which crystallized on seeding with a crystal of material obtained from the first coupling described earlier, and then i'-a-~-ribofuranosyladenine ( I I ) , purified through its picrate. Properties of the anomeric nucleosides and some of their precursors are given in Table I , along with those of the nucleoside from pseudovitamin B12, adenosine, and its a-anomer. The samples of the &anomer I prepared by the two different coupling reactions were identical, whereas the properties of the a-anomer I1 are in good agreement with those reported for the nucleoside from pseudovitamin Biz. The optical rotations of the anomers (@, -84.9 0.2'; a , 0") confirm the assignment of a- and @-configurationsand agree well with the relative values for adenosine and its a-anomer (-60.4' and f24').
*
(9) B. R . B a k e r , Ciba Found S y m o . , C h e m . Bioi. Purines, 120 (1957). ( I O ) S o t r u e exceptions t o t h e trans rule have been reported in t h e s y n thesis of purine nucleosides 11 I n rare instances a minor a m o u n t of a - a n o m e r has been d e t e c t e d ' ? b u t t h e @-anomeris always t h e principal product of t h e reaction I n our reaction only one anomer could be detected. (11) J . A. Montgomery a n d H J . T h o m a s , A d v a n . Carbohydrate C h e m . , 17, 301 ( 1 9 0 2 ) . (12) B R . B a k e r , R E . S c h a u h , a n d H. M . Kissman, J. A m . Chem. Soc., 7 7 , 3911 (195.51; H 51 K i s s m a n a n d B R B a k e r , ibid.,79, ,5534 ( 1 9 5 7 ) . 113) This ribose derivative. which contains n o acyloxy group a t C-2 t o cause stereospecific entry n i a purine a t C-19 giving rise t o a @-ribonucleoside only, has been u i e d t o prepare a mixture of adenosine a n d its a - a n o m e r ' 4 (14) R S Wright. G M . T e n e r , a n d H. G K h o r a n a , J . A m . Chem Soc., 80, 2004 (1958).
I t is now possible to state with certainty t h a t the nucleoside moiety of pseudovitamin BIZis in fact 7 - c ~ ~ ribofuranosyladenine. Synthesis of the nucleosides from other BIZvitamins is in progress. KETTERISG-MEYER LABORATORY
SOUTHERN RESEARCH INSTITUTE
A. MOSTGOMERY H. JEANETTE THOMAS
JOHN
BIRMINGHAM, ALABAMA RECEIVED JULY22, 1963
A Two-Stage, Two-Center Decarboxylation' Sir: The elusive question of simultaneity in multicentertype reactions continues to receive its most critical examination in studies of Diels-Alder reactions.2 Suggestions that a range of transition state structures might be accessibleZb,'are supported by results which clearly demonstrate the polar ambivalence of such states3 and warn that classical elucidative techniques might well distort the very phenomena they seek to m e a s ~ r e . The ~ dilemma can be avoided by isotopic substitution, preferably a t the reaction sites but a t adjacent atoms. Interpretations of these latter, a-hydrogen isotope effects have been rendered ambiguous by the discovery6a of one clear exception to what had been regarded6bas a universal, though necessarily empirical pattern. Less equivocal decisions should follow from determination of primary isotope effects a t both reaction sites and we here report the first such investigation in this area.' Thermal decarboxylation of the a-pyrone-maleic anhydride adduct8 was chosen for study assuming (a) t h a t this might legitimately be regarded as a DielsAlder retrogression and (b) that the structural sim( I ) T h e s t u d y was supported by t h e Air Force Office of Scientific R e search, under C o n t r a c t S o .49(038)-942 a n d G r a n t No. 142-63 (2) (a) R B Woodward a n d T . J . K a t z , Tetrahedron, 6, 70 (1959), (h) J. A. Berson a n d A. Remanick, J A m . C h e m . SOL.,83, 4947 (1961); (c) C . Walling a n d H . J Schugar, ibid , 86, 607 (1963); ( d ) S Seltzer, ibid., 86, 1360 (1963) (3) J . Sauer a n d H. Wiest, Angew Chem , 74, 353 (1962) (4) T h i s has been clearly recognized in t h e s t u d y of r a t e dependence on pressure ( C Walling a n d D . D. T a n n e r , J A m . C h e m . Soc., 86, 612 (1963)). (5) ( a ) D . E . Van Sickle, Tetrahedron Letters, 687 (19613; ( b ) S. Seltzer, ibid., 457 (1962) (6) (a) R . E. Weston, J r , a n d S . Seltzer, J . P h y r . Chem , 68, 2192 ( l 9 6 2 ) , (b) S Seltzer, J . A m . Chem Soc , 83,202.5 (1961). ibid., 86, 14 (1963). (7) ( a ) G A . R o p p , V F R a e n , and A. J . Weinherger, ibid , 7 6 , 3697 (19.53); ( b ) K . F. W. Bader a n d A N.Bourns, Can J Chem , 3 9 , 3 4 8 (1961), are t h e most closely related studies in object (a) a n d approach ( b ) (8) ( a ) 0 Diels and K Alder. A n n Chem., 4 9 0 , 2.57 (1931); (h) B R L a n d a u , P h D . Thesis, H a r v a r d University. 1960
