Silyl enol ethers in Paternò‐Büchi reactions. Functional group tolerance and the effect of β‐alkyl substitution

The photochemically induced diastereoselective formation of 3‐(silyloxy)oxetanes from silyl enol ethers and aromatic aldehydes was studied. It is shown that many heteroatom‐ and double bond‐containing substituents (ether, ester, acetal, amide, alkene) withstand the reaction conditions and may therefore be attached either to the carbonyl compound (e.g. 9) or to the silyl enol ether (e.g. 1). The formation of several functionalized oxetanes such as 2 and 11 was achieved in decent yields (42–70%). Furthermore the effect of β‐substituents at the silyl enol ether was investigated. A stereoconvergent Paternò‐Büchi reaction of aromatic aldehydes afforded the corresponding oxetanes (13, 16–18) in fair to excellent yields (45–87%) and with diastereoselectivities (ds) of 65–95%. No detrimental influence of the steric bulk in the β‐position (R1 = Me, Et, and iPr) on the regiochemistry of the reaction was observed. Large substituents in the α‐position of the enol ether [R = iPr, tBu, C(OCH2)2Et, CMe2CHCH2] proved beneficial, isolated yields of the major diastereoisomeric oxetane ranging between 60 and 87%. The configuration of the products was elucidated by 1H‐NMR spectroscopy. In the major isomer all large vicinal substituents Ar (at C‐2), R (at C‐3), and R1 (at C‐4) at the oxetane nucleus are oriented trans to each other. The stereoselective formation of three stereogenic centers at a time can be explained by a non‐concerted mechanism which invokes the intermediacy of 1,4‐diradicals as intermediates.