Scheme 23 Synthesis of (£)-basidalin mimic to the biogenetic route [113]

For the preparation of 4-amino-2(5ff)-furanone as above, the disfavored outward rotation of hydroxyl group was compensated for by adding an acid such as trifluoroacetic acid to assist cyclization [114]. Notably, 4-aryl-4-hydroxy-2-cyclobutenone bearing o-carboranyl substituent at C-2 also gave rise to the corresponding 2(5ff)-furanone (124 ^ 125) rather than the usual quinone, indicating that direction of rotation is affected even by substituent at C-2 [115]. Anyhow, the product has potential utility for boron neutron therapy. Analogously, in a cyclobutenone system other than squaric acid, intramolecular addition of hydroxyl group to in situ formed ketene was also reported to give a lactone ring (126 ^ 127) [116] (Scheme 24).

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