Some aspects of organophosphorus chemistry
An evaluation is made of ethyl (diethoxyphosphino)-acetate as a common intermediate for the synthesis of various phosphinoline and isophosphinoline systems. Two syntheses of 2-hydroxy-l,2,3,4-tetrahydroisophosphinoline 2,4-dioxide from [special character omitted]-bromotoluene and 2-(bromomethyl)benzoic acid are described. Attempts to induce cyclisation of 3-(hydroxyphenyl-phosphinyl)propanoic acid with polyphosphoric acid, and the corresponding bis-acid chloride with aluminium chloride, prove unsuccessful, due to deactivation of the ring towards electrophilic attack. Three 1-substituted-1,2-dihydrophosphinoline 1-oxides are synthesised, which when photolysed in a nucleophilic solvent give the three double-bond isomers of l-(2- phosphorylphenyl)propene derivatives. This is rationalised in terms of solvent trapping of a phospha-quinoidal intermediate, obtained by electrocyclic ring-opening of the phosphinoline. The Lewis acid catalysed cyclisation of 2- biphenylylphosphorodichloridothioate to 6-chloro-6H-dibenz (c,e)(1,2)oxaphosphorin 6-sulphide is investigated under a variety of conditions, leading to the identification of the optimum procedure as a neat reaction at 200 C for Ih, in the presence of 0.1 equivalent of aluminium chloride. Esterification of the resultant heterocycle, followed by photolysis of a methanolic solution gives three unidentified products. The complete absence of any photolytic reaction of the analogous 9,10-dihydro-9-methoxyphosphaphen anthrene leads to the conclusion that reaction is not by a concerted, electrocyclic pathway. t-Butylamine is found to selectively monodemethylate phosphorus esters in the presence of carboxylate esters in a side-chain. Marked preference for removal of a methyl over benzyl or higher alkyl groups is demonstrated by reactions with mixed esters. Treatment of dimethyl 2-bromoethylphosphonate with t-butylamine, however, gives dimethyl vinylphosphonate, while dimethyl 3-bromopropyl-phosphonate undergoes nucleophilic substitution. The mildness of conditions required to demethylate a phosphorus ester leads to the suggestion of using methyl as phosphate protecting-group in nucleotide synthesis.