Synthesis of chlorophyll‐ a skeleton homologs using grignard reaction with methyl pyropheophorbide‐ a

From methyl pyropheophorbide‐ a (MPPa) (1), methyl 2‐formylmethyl‐2‐devinyl‐9‐ethylenedioxy‐9‐deoxopyropheophorbide‐ a (2) and methyl 3‐acetyl‐9‐ethylenedioxy‐2‐devinyl‐9‐deoxopyropheophorbide‐ a (3) were prepared. The Grignard reactions of 2 and 3 were performed using cycloalkyl magnesium bromides to afford cycloalkyl‐substituted sec ‐alcohol 4 and tert ‐alcohols 9a‐c, respectively. By the deprotection of the ethylenedioxy group, these alcohols were respectively converted to exocyclic ketones 5 and 10a‐c, which were dehydrated to give chlorins 6 and 11a‐c having an alkenyl function at the 2‐position. On the other hand, the oxidation and deprotection of the alcohol 4 gave a diketo chlorin 8.