Chapter 12
Stereoselective Synthesis of Carbohydrates from Acyclic Precursors
Downloaded by PENNSYLVANIA STATE UNIV on May 21, 2013 | http://pubs.acs.org Publication Date: December 30, 1989 | doi: 10.1021/bk-1989-0386.ch012
R. R. Schmidt Fakultät Chemie, Universität Konstanz, D-7750 Konstanz, Federal Republic of Germany
Inverse type hetero-Diels-Alder reactions between β - a c y l o x y - α - p h e n y l t h i o substituted a, β-unsaturated cabonyl compounds as 1-oxa1,3-dienes, enol ethers, α-alkoxy a c r y l ates, and styrenes, respectively, as hetero-dienophiles result in an efficient one step synthesis of highly functionalized 3,4-dihydro-2H-pyrans (hex-4enopyranosides). These compounds are diastereospecifically transformed into deoxy and amino-deoxy sugars such as the antibiotic ramulosin, in pyridines having a variety of electron donating substituents, in the important 3-deoxy-2-glyculosonates, in precursors for macrolide synthesis, and in C-aryl-glucopyranosides. Carbohydrate d e r i v a t i v e s and r e l a t e d n a t u r a l products are mainly synthesized by transformation o f r e a d i l y a v a i l a b l e sugars ( 1 - 2 ) . However, the high d e n i s t y o f func t i o n a l groups o f comparable r e a c t i v i t y r e q u i r e s r e g i o selective protection and d e p r o t e c t i o n measures arid s t e r e o s p e c i f i c f u n c t i o n a l group exchange r e a c t i o n s which o f t e n r e s u l t i n m u l t i s t e p syntheses. Several methods have been developed over the l a s t years which are u s e f u l i n the d i a s t e r e o s e l e c t i v e and the e n a n t i o s e l e c t i v e gene r a t i o n o f new stereocenters (4.) . Therefore "de novo syn theses" or " t o t a l syntheses" o f carbohydrate d e r i v a t i v e s and r e l a t e d n a t u r a l products from a c h i r a l s t a r t i n g mate r i a l s may be competitive or even s u p e r i o r (5.-10.) · NOTE: This chapter is part 31 in the series, "De Nnvo Synthesis of Carbohydrates and Related Natural Products". 0097-6156/89/0386-0182$06.00/0 * 1989 American Chemical Society
In Trends in Synthetic Carbohydrate Chemistry; Horton, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
Downloaded by PENNSYLVANIA STATE UNIV on May 21, 2013 | http://pubs.acs.org Publication Date: December 30, 1989 | doi: 10.1021/bk-1989-0386.ch012
12.
SCHMIDT
Stereoselective Synthesis from Acyclic Precursors
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For the generation o f s e v e r a l contiguous c h i r a l cen t e r s , two a l t e r n a t i v e key step r e a c t i o n s can be a p p l i e d : (i) s t e r e o s e l e c t i v e CC-bond formation; and ( i i ) stereo s e l e c t i v e f u n c t i o n a l i z a t i o n o f compounds having already the r e q u i r e d carbon skeleton. In the de novo-synthesis of carbohydrates and r e l a t e d n a t u r a l products v i a i n v e r s e type h e t e r o - D i e l s - A l d e r r e a c t i o n s both o f these p r i n c i p l e s are e s p e c i a l l y w e l l documented as w i l l be shown below (8-10). Aiming at the pyranose form o f sugars, normal type hetero-Diels-Alder r e a c t i o n s were e x t e n s i v e l y used f o r the synthesis o f f u n c t i o n a l l y s u b s t i t u t e d dihydropyran and tetrahydropyran systems (5-10) (see routes A - D i n the general Scheme 1) which are a l s o important t a r g e t s i n the "chiron approach" t o n a t u r a l product syntheses (3.) . H e t e r o - D i e l s - A l d e r r e a c t i o n s with inverse e l e c t r o n demand such as a, β-unsaturated carbonyl compounds (l-oxa-1,3dienes) as heterodienes and enol ethers as heterod i e n o p h i l e s , are an a t t r a c t i v e route f o r the synthesis of 3, 4-dihydro-2u-pyrans (H) .
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