While the addition of dichlorocarbene to 2, 5-dihydro-3-methyl-lH-phosphole 1-oxides (la-d) carried out under phase-transfer catalytic conditions gave 6, 6-dichloro-l-methyl-3- phosphabicyclo [3. lO] hexane 1-oxides (za-d), the 3, 4-dimethyl-l-phenyl analogue (3a) yielded, via four consecutive steps including two ring expansions, 4, 5-dichloro-3, 6-dimethyl- l-phenylphosphacycl~ hepta-2, 4, 6-triene 1-oxide (7a). In the reaction of 2, 5-dihydro-l- ...