Phosphorus compounds. XCVII: Synthesis of polycyclic phosphorus cage compounds containing diphosphirane and phosphirane units by tandem Diels-Alder and ene reactions : a contribution to the cycloaddition and enophile chemistry of phosphaalkynes

When the open-chain 1,3-dienes 14a-k are allowed to react with the phosphaalkyne 13 in a molar ratio of 1 :2 under thermal conditions, the 1,7-diphosphatricyclo[3.2.1.0 2,7 ]oct-3-enes 19 and/or 20 are formed. The formation of isomers is attributable to the initial Diels-Alder reaction since both the subsequent ene reaction (specific formation of 17, 18) and the concluding intramolecular Diels-Alder reaction (→19, 20) proceed specifically. The constitutions of the tricyclic systems incorporating a diphosphirane unit were confirmed by NMR spectroscopic data and a crystal structure analysis of 19a (= 20a). The s-cis-configurated 1,3-butadienes 141-u react similarly with 13 (Diels-Alder reaction → phospha-ene reaction → intramolecular Diels-Alder reaction) to furnish the tetracyclic products 191-u (= 201-u). The same reaction sequence is also responsible for the regiospecific formation of the diphosphatetracyclic systems 19v-y from the reactions of 13 with the semicyclic 1-vinylcycloalk-l-enes 14v-y. In principle, phosphaalkynes such as 13, independent of the above-mentioned reaction sequence, are suitable for use as enophiles in ene reactions. This is illustrated by the conversions 21 + 13 → 22 and 21 + 22 → 23. The cyclohexa-1,4-diene 24a reacts with 13 to furnish the phosphatricyclooctene 29a, a result in complete harmony with the reaction sequence 15/16 → 19/20. Product 29a now contains a phosphirane unit instead of the diphosphirane in the former substances. The regioselectivity of the ene reaction is lost when the substituted 1,4-dienes 24b-e and the annelated derivatives 24f,g are used as reaction partners for 13. The cyclohexa-1,4-diene 24b participates in a completely nonspecific reaction with 13 (→ 29b, 30b, 31b, 32b).