2,3-Dihydro-4H-pyran and various 2-substituted 2,3-dihydro-4H-pyrans have been shown to react with certain 6-substituted purines in the presence of a catalytic amount of acid to give the corresponding 6-substituted-9-(tetrahydro-2-pyrany1)-purines. This reaction provides a new synthetic route to the preparation of valuable models of certain purine deoxynucleosides which possess significant antitumor activity. The tetrah ydropyran molecule greatly increases the s o hbility of purine derivatives in organic solvents and provides a useful blocking group which can be readily used to prevent undesirable side reactions due to the presence of the imidazole hydrogen. Acid hydrolysis regenerates the purine.( 5 ) The synthetic route employed in the synthesis of N-methylalloxan and of N-phenylalloxan start~ nlth the appropriate monosubrtituted urea and cyano.cetic-corboxyl-C'~ acid to yield RNH-CONHC*OCHsCN, which undergoes ring closure in the pruence of
The utility of the vitamin C and isovitamin C derived dibromo diacetates 3a,b for preparing synthetically useful chirons is further examined. Methodologies for transforming the readily accessible compounds 3a,b into 3,6-and 5,6-anhydrohexono-l,4-lactones lOa,b and 6a,b are presented. Elaboration of the lactone epoxides 6a-c has provided optically pure (S,S)-, meso-, and (R,R)-4,4'-bis-y-butyrolactones 18a-c. The difference in reactivity between some isomeric intermediates was related to the spatial arrangement a t the reactive sites.
IntroductionUnequivocal stereocontrol constitutes the center piece of modern natural product synthesis. Most frequently the absolute configuration of the desired product has stemmed from the judiciously timed incorporation of chirally defined building blocks (chirons) during the synthesis and the subsequent configurational control of the asymmetric centers introduced. Carbohydrate-based schemes leading to a host of chirons have been especially rewarding in this respect; most of the desired routes have relied on the restructuring of furanoside and pyranoside systems.Our recent studies have demonstrated some relevant advantages of utilizing aldono-1,4-lactones, rather than the corresponding lactols, towards these ends. In practice aldono-1,4-lactones are highly crystalline, easily manipulated, and readily characterized substances. Large amounts of starting lactones may be prepared from inexpensive, commercially available materials. Publications from these laboratories have described the production of chirally defined butenolides 2 a -k '~~ from the ascorbic acids la,b and from ~-ribono-1,4-lactone. The preparation of three enantiomerically pure 4,5,6-trihydroxy-nor-~-leucines from la,b has also been r e p~r t e d .~
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