VISIBLE-LIGHT INITIATED PHOTOSENSITIZED ELECTRON-TRANSFER CYCLIZATIONS OF ALDEHYDES AND KETONES TO TETHERED ALPHA,BETA-UNSATURATED ESTERS -STEREOSELECTIVE SYNTHESIS OF OPTICALLY PURE C-FURANOSIDES

Photosensitized one-electron reductive activation of aldehydes/ketones tethered with activated olefins leads to efficient cyclization to giv e diastereoselective cycloalkanols in high yield. The activation is pr omoted by secondary and dark electron transfer from visible light (405 nm) initiated photosensitized electron transfer generated 9,10-dicyan oanthracene radical anion (DCA(.-)). The DCA(.-) is produced by electr on transfer using either triphenylphosphine (Ph3P) as sacrificial elec tron donor (PS-A) or 1,5-dimethoxynaphthalene (DMN) as primary electro n donor and ascorbic acid as sacrificial electron donor (PS-B), to lig ht-absorbing DCA. The cyclization is suggested to involve ketyl radica l intermediate. High trans diastereoselectivity is observed during the formation of cycloalkanols. This cyclization strategy is further exte nded for the stereoselective synthesis of optically pure C-furanoside (41), starting from naturally occuring L-tartaric acid. The stereochem istry of 41 is suggested based on the single-crystal X-ray diffraction data.