Лако-красочные материалы — производство

Use of the Claisen rearrangement achieves the same conversion of methylbutenol into methylheptenone as does the Carroll reaction, but without the loss of carbon dioxide. The methanol produced instead can be recovered and recycled. In this process, as shown in Scheme 4.7, methylbutenol is treated with the readily available 2-methoxypropene to give the allyl vinyl ether, which then undergoes a Claisen rearrange­ment to give methylheptenone.

Prenyl Chloride

Addition of hydrogen chloride to isoprene gives prenyl chloride, together with some of the allylic isomer 2-chloro-2-methylbut-3-ene. The presence of this tertiary chloride is not deleterious since, whilst prenyl chloride reacts with acetone via an SN2 reaction, it reacts by an Sn2′ mechanism to give the same product, methylheptenone (Scheme 4.8). Further elaboration to linalool, etc., is the same as in the processes
described above. This process has been operated by Rhone-Poulenc in France and by Kuraray in Japan.

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Isoprene

Methylheptenone

Scheme 4.8

The Ene Reaction

Aldol condensation of acetone with formaldehyde gives methyl vinyl ketone. This can undergo the ene reaction with isobutylene to give an isomer of methylheptenone, 2-methylhept-l-en-6-one. Isomerization to methylheptenone is easy using an acidic catalyst, and acetylene can be added to either isomer. From methylheptenone, the process leading to linalool is the same as in the examples above. If the acetylene is added to 2-methylhept-l-en-6-one, then isodehydrolinalool results. This is actually advantageous when the monoterpene unit is to be used as a precursor for ionones and vitamins, since the 1,1-disubstituted double bond is more reactive in the cyclization reaction than is the normal 1,1,2-trisubstituted bond. (The cyclization reaction is described in detail below.) The preparation of these two acetylenic ketones is shown in Scheme 4.9 and is the basis of a process commercialized by BASF.