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17.8.1.1.3.2.1 Variation 1: 1,19-Dimethyl-b-bilenes

DOI: 10.1055/sos-SD-017-01748

Smith, K. M.; Vicente, M. G. H.Science of Synthesis, (2004171106.

1,19-Dimethyl-b-bilenes can be prepared by two general methods (Scheme 25). The first is suited to the synthesis of fully alkylated bilenes, but gives low yields when bilenes bearing electron-withdrawing substituents are required; condensation of 2 equivalents of a 2,3,4-trialkyl-1H-pyrrole (e.g., 74) with one of a 1,9-bis(methoxymethyl)dipyrromethene 111 affords a b-bilene salt 112, which is usually symmetrical about the 10-carbon atom.[‌70‌‌73‌] Better is the route in which a 1-formyldipyrromethane 113 is treated with a 1-unsubstituted dipyrromethane (or with the corresponding 1-carboxylic acid) 114;[‌74‌] in this way unsymmetrically substituted b-bilenes 115 can be obtained in high yield, but it is advisable[‌75‌] to have an electronegative group on the terminal rings of the b-bilene in order to prevent acid-promoted side reactions which result in mixtures of porphyrins.

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