Synthetic potential of cycloalkene - derived phosphonates

Abstract

The effect of the diethoxy phosphoryl group on an adjacent carbon-carbon double bond was determined for various unsaturated systems and found to be very similar to that of the hydrogen atom. The number of carbon atoms attached to the double bond and allylic hydrogen atoms were found to play the determining role in olefin stabilisation of alkenylphosphonates. The alkylation reactions of 1, 2- and 2, 3-unsaturated cyclohexenylphosphonates were also investigated. Diethyl cyclohexen-1-ylphosphonate failed to react but both diethyl cyclohexen- 2-yl- and cyclohexen-1-ylmethylphosphonate were usually alkylated in the a-position with respect to phosphorus. One example of -alkylation was described. Aldehyde addition was also found to occur via the a-carbon. Introduction of sterically bulky groups on either the electrophilic carbonyl centre or the a-position on the nucleophile led to diminished yields and to -addition being favoured. The latter is a consequence of reversibility of the addition step. Hence, thermodynamic products could be isolated. The synthetic viability of diene formation from the 2-hydroxy adducts, obtained from the aldehyde addition reactions, was investigated. Although the retro-addition reaction competed with fragmentation to dienes, reasonable to high yields of the latter were obtained. A proton nuclear magnetic resonance spectroscopic study, as well as X-ray crystal structure determinations, of the 2-hydroxyphosphonates were conducted in order to establish the reasons for the observed difference in reactivity between the RR (SS) and RS (SR) diastereoisomers in their fragmentation reactions.