Unexpected regioselective diels-alder cycloaddition reactions between 3-fluorobenzyne and 2-alkylfurans

The Diels-Alder reaction between 3-fluorobenzyne (1) and 2-alkylfurans (2) gives a mixture of syn adduct 3 and anti adduct 4. The syn cycloadduct invariably predominates and the regioselectivity increases in the order: R = Me < Et < i-Pr < t-Bu, to a maximum of 90% syn adduct 3d for 2-tert-butylfuran. These results are rationalized in terms of a concerted nonsynchronous transition state that reflects an alkyl steric effect and a polarized aryne 1.