CHEMISTRY OF THE HERBICIDINS - REACTIVITY OF SILYL ENOL ETHERS DERIVED FROM SIMPLE AND CARBOHYDRATE-BASED TETRAHYDROPYRANS

Details are described of preliminary synthetic studies, based on Lewis acid-mediated alkylation of a silyl enol ether, that were directed to wards the C-11 glycoside of the herbicidin class of nucleosides. The c hemistry presented focuses on limitations encountered with the reactiv ity of both the electrophilic and nucleophilic components designed to serve this longer term synthetic objective. alpha-Chloro sulfide 6 rea dily undergoes a Lewis acid-promoted internal redox reaction leading t o sulfide 8; this is a consequence of the O-benzyl protecting group us ed at C-3 of chloro sulfide 6. This pathway is avoided by use of O-sil yl protection, and reaction of the silyl-protected alpha-chloro sulfid e 11 with the simple heterocyclic silyl enol ether 5 gives the herbici din models 13a and 13b incorporating the required furano-pyrano-pyran skeleton. Further experiments showed that the nucleophilic component r equired for the herbicidins, the carbohydrate-based silyl enol ether 2 , readily underwent Lewis acid-mediated rearrangement to give levogluc osenone 15.