[4+2] and [2+2] photocycloadditions of 1,2-diketones to glyeal and hydroxyglycal esters

Photocycloadditions of phenanthrenequinone 1 and acenaphthenequinone 16 to 3,4,6-tri-O-acetyl-D-glucal proceeded with distinctly different regioselectivities: 1 preferentially adds with both carbonyl oxygens to give the [4+2] cycloadduct, a 1-O,2-O-annelated alpha-D-glucoside (50% [Chem. Ber. 1952, 85, 531]), while 16 reacts with only one carbonyl group to exclusively yield the [2+2] addition product, a 1-C,2-O-oxetano-alpha-D-glucoside (86%). 2-Hydroxyglucal esters 11 and 12 also undergo photoadditions with 1 to give the cis-1,4-dioxane-fused cycloadducts 13 and 14, whilst 16 fails to react. Structural and configurational assignments rest on NMR data and an X-ray analysis of spiro-pyranooxetane 18. (C) 2004 Elsevier Ltd. All rights reserved.