Efficient Synthesis of 2-Deoxy l-Ribose from l-Arabinose:  Mechanistic Information on the 1,2-Acyloxy Shift in Alkyl Radicals

[formula: see text] Conversion of the inexpensive L-arabinose 1 into the ethylthio ortho ester 7 followed by generation of the dialkoxyalkyl radical III produces the desired 2-deoxy-L-ribose triester 4 in excellent overall yield. It has been shown that the similar dialkoxyalkyl radical IV is not an intermediate in the 1,2-acyloxy shift of anomeric radical I.