Citral enol acetate

Routes to furan monoterpenes from other monoterpenoids are well known, and the photooxygenation route from myrcene (7) to perillene (849) was discussed in Vol. 4 (p. 561). Using the aldehyde 863 led, by the same route ( 62, then ferrous ion) to perillenal (846). The simplest reported preparation of perillene (849) from a dimethyloctane monoterpenoid precursor is certainly the autoxidation of citral enol acetates (864), yielding 11% of a mixture of perillene (849) and rose furan (848) after passing air through a chloroform solution for 24 hours. 

Simple aliphatic or aromatic ketones are not suitable substrates in the Pericin transformation. Similarly, aliphatic aldehydes are not generally acceptable components in this reaction. This long-known limitation was studied by Crawford and Little. They demonstrated that many aliphatic aldehydes give diacetates and enol acetates when heated with acetic anhydride, with or without sodium acetate, suggesting this side reaction causes the typical Perkin process to fail. Semmeler had previously obtained similar results. They did show, however, that several short chain aldehydes, as well as frowj-citral (16), undergo the Perkin condensation with p-nitrophenylacetic anhydride the p-nitrophenyl substituent activates the anhy-... 

Synthesis from Isobutene and Formaldehyde. 3-Methyl-3-buten-l-ol, obtained from isobutene and formaldehyde [43], isomerizes to form 3-methyl-2-buten-l-ol [58]. However, it is also converted into 3-methyl-2-butenal by dehydrogenation and subsequent isomerization [59], [60]. Under azeotropic conditions in the presence of nitric acid, 3-methyl-2-buten-l-ol and 3-methyl-2-butenal form an acetal (shown below) [61], which eliminates one molecule of 3-methyl-2-bu-ten-l-ol at higher temperatures. The intermediate enol ether undergoes Claisen rearrangement followed by Cope rearrangement to give citral in excellent yield [62] ... 

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