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28.6.1.1.2.2 Variation 2: Photochemical Cyclization

DOI: 10.1055/sos-SD-028-00503

Echavarren, A. M.; Porcel, S.Science of Synthesis, (200628513.

An analogous route to Section 28.6.1.1.2.1 uses the cyclization of benzoins induced by light. This route was first described in 1978,[‌53‌] and has only been applied to symmetrically substituted benzoins, which are available from the corresponding benzaldehydes. The photocyclization is performed on a derivative 11 of the benzoins with a fixed cis configuration in order to avoid cis/trans isomerization which would decrease the yield of the ring closure. The cis configuration can be fixed: by forming a carbonate 11 (X=CO) by treatment with phosgene,[‌53‌] a 2,3-dihydro-1,4-dioxin 11 (X=CH2CH2) by treatment with ethylene glycol,[‌54‌] or a dioxaborole 11 (X=BPh) by treatment with phenylboronic acid.[‌54‌] Finally irradiation of 11 with UV light gives a ring-closed intermediate 12 that is further oxidized in situ to 13 and then to the 3,6-substituted phenanthrene-9,10-dione 14 (Scheme 7).

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References


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