Pergamon Tetrahedron 55 (1999) 2157-2170 TETRAHEDRON Chiral 1,4-Dicarbonyl-2,3-O-Isopropylidene Derivatives. Rapid Racemization on Standing Juan B. Rodriguez Departamento de Qulmica Orgtlnica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabelltn 2, Ciudad Universitaria RA-1428 Buenos Aires, Argentina Received 22 September 1998; accepted 22 December 1998 Abstract 1,4-Dicarbonyl-2,3-chiral derivatives are useful synthetic precursors for the preparation of carbocyclic tings but, in many cases, losses of optical purity have been reported. 1-Deoxy-3,4-O-isopropylidene-6-O-trityl- D-erythro-hexo-2,5-diulose and 1-deoxy-3,4-O-isopropylidene-6-O-(tert-butyldiphenylsilyl)-D-erythro-hexo- 2,5-diulose were synthesized from D-ribono-l,4-1actone. These compounds were selected to study the epimerizability of 2,3-O-isopropylidene-1,4-dicarbonyl derivatives. It was found that both compounds smoothly epimerize and partially racemize on standing. © 1999 Elsevier ScienceLtd. All rights reserved. Keywords: Carbohydrates; Nucleosides; Racemization; Epimerization INTRODUCTION Carbohydrates are an important chiral source for the preparation of enantiomerically pure compounds [1,2]. Of special interest is the synthesis of carbocyclic rings for being advanced PGO ~ ,, L._._./';'OH =-" = o) o 1, PG = Bn 2, PG = MOM 3, PG = Ph2tBuSi precursors of carbanucleosides, compounds in which the furanosic oxygen has been replaced by a methylene group, giving rise to a new family of nucleoside analogues [3,4]. As normal nucleosides, these compounds exhibit a wide range of biological properties with the advantage that they are not recognized by phosphorylases and hydrolases that cleave the glycosidic bond [5,6]. The availability of optically pure carbocyclic nucleosides is crucial to evaluate their biological action, mainly as antiviral and antitumor agents. The ° Fax: (54 1) 576-3346. E-mail: jbr@qo.fcen.uba.ar 0040-4020/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PH: S0040-4020(99)00007-1