The density functional theory level (B3LYP/6-311G * * ) computations of the Diels-Alder (DA) reactions of 5,6-dihydrothiazolo[3,2-d][1,4,2]diazaphospholes with 1,3-butadiene and with isoprene confirm pericyclic mechanism via asynchronous transition states. The aromatic character of the transition states is established by negative nucleus independent chemical shift (NICS) values falling in the range from -14 to -16. Integration of the dienophilic >C P functionality in the 6π aromatic azaphosphole ring raises the activation energy barrier (B3LYP/6-311++G * * //B3LYP/6-311G * * ) compared to that for the DA reaction of the acyclic phosphaethene, but it is lower than the activation energy barrier for the DA reaction of the corresponding 10π aromatic system, thiazolo [3,2-d][1,4,2]diazaphospholes. The experimentally observed stereo-and regioselectivities in the reactions can be accounted on the basis of secondary molecular orbital (SMO) interactions detected in the respective transition structures. The attachment of an electron-withdrawing group to the dienophilic moiety enhances both stereo-and regioselectivities which agree well with the experimental values. Solvent (toluene) effect studied with polarizable continuum model (PCM) indicates that the stereo-and regioselectivities are not affected by the sol-