Bicyclo[3.2.0]hept-6-ene (1a) and 2,4-dioxabicyclo[3.2.0]hept-6-ene (1b) reacted with diazomethane and phenylglyoxylonitrile oxide to give only anti adducts. In contrast, a mixture of syn and anti adducts was isolated from the reactions of the same 1,3-dipoles with bicyclo[3.2.0]hept-6-en-3-one (1c), 2,4-dioxabicyclo[3.2.0]hept-6-en-3-one (1e), and even with the apparently most crowded, on the ... [Show full abstract] syn face, 3,3-dimethyl-2,4-dioxabicyclo[3.2.0]hept-6-ene (1d). Extensive ab initio MO calculations (4-31G) showed that compounds (1) prefer a boat-like conformation which, however, becomes flatter and flatter on passing from (1a) to (1e). As a result there is a progessive lessening of steric hindrance on the syn face which neatly parallels the observed increase in syn attack along the series (1a–e). Moreover the energy required to remove steric hindrance on passing from the boat-like to the half-planar conformation is definitively lower for (1c–e) than for (1b and a).