Oxaphospholic cyclization

Keywords 1,2-Alkadienephosphonates 1,2-Alkatrienephosphonates 1,2-Oxaphosphole Oxaphospholic cyclization... 

The cyclic phosphonium salts 140,141,143,145, and 146 so obtained are evidence for the mechanism of the oxaphospholic cyclization and especially for the main role of the tertiary carbocation formation during the process. The additional data which support this assumption, come from the investigation of the same reaction, but with different substrate, i.e., dimethyl(l,2-hexadienyl)phosphine oxide 147. In this case, the reaction mechanism involved formation of secondary carbocation that gives oxaphosphole product 148 only in 10% yield (Scheme 60) [124],... 

The mechanism of oxaphospholic cyclization was confirmed by isolating 152 in the reaction of /V-methyl-/V-propargylamine with dialkylchlorophosphite (Scheme 61) [125],... 

An attempt to obtain other evidence for the mechanism of the oxaphospholic cyclization has been made and is shown in Scheme 76 [152],... 

All the synthetic protocols described above have limitations to some extent and the yields of the products were modest. In some cases the formation of 1,2-alkadi-enephosphonate derivatives is essential for obtaining the final cyclic products. This is the reason why many authors have used the higher reactivity of 1,2-alkadi-enephosphonates, discovered by Mark [42] in 1962 for the preparation of 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives. Since then, the oxaphospholic cyclization of 1,2-alkadienephosphonate system of double bonds has become the easiest method for the synthesis of these compounds. The special structure of phosphorylated allenes is responsible for their special properties, which has attracted the attention of chemists for a long time [43 16],... 

It was suggested that the reaction of 3-methyl-1,2-butadienephosphinic acid with bromine in water involved oxaphospholic cyclization and oxidation of the substrate (Scheme 18) [58],... 

Compounds 53 could be obtained from an acid-promoted oxaphospholic cyclization of phosphinic acid 52 in the presence of mineral acid and heating up to 100 °C (Scheme 21) [41]... 

Regio- and stereoselective cyclization-Heck reaction of monoesters of 1,2-allenyl phosphonic acids with alkenes afforded 4-(1 (Z)-alkenyl)-2-ethoxy-2,5-dihydro-1,2-oxaphosphole 2-oxides (eq 115). ... 

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