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Organic Lab Reactions 23 - been developed to give a good...

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18 THE CLAISEN REARRANGEMENT provides a convenient method of introducing a propyl group into a phenol. Because of the occurrence of inversion, compounds of the structure HOArCH 2 CH=CHR cannot be prepared by rearrangement of ethers containing substituted allyl radicals such as farnesyl and phytyl groups. /3-Methylallyl ethers are not subject to this disadvantage, because inversion does not change the structure of the group, and rearrangement of the ethers followed by reduction has been employed as a convenient method of introducing the isobutyl group into phenols. 66 The allyl group in the allylphenols can be oxidized, after protecting the hydroxyl group, to yield substituted phenylacetaldehydes 73> 76>76 and phenylacetic acids. Thus, homogentisic acid LXIII is prepared readily by ozonizing the dibenzoate of allylhydroquinone LXIV, which is obtained by rearrangement of the allyl ether of hydroquinone mono- OH OCH 3 OH LXIV u CH2CHO LXV benzoate followed by benzoylation. 77 The ozonization procedure has
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Unformatted text preview: been developed to give a good yield of 3,4,5-trimethoxyphenylacetalde-hyde (LXV) from the corresponding allyl compound. 73 The 2-allylphenols in the presence of acid catalysts such as pyridine hydrochloride,*' 66 hydrobromic acid-acetic acid, or forniic acid 36 form 2-methyldihydrobenzofurans (coumarans) such as LXVI. In the pres-O CHCH 3 C(CH 3 ) 2 •CH 2 LXVI LXVII LXVIII ence of hydrogen bromide and a peroxide, 2-allylphenyl acetate gives the isomeric dihydrobenzopyran or chroman (LXVII). 29 ' 78 ' t Ring * See p. 26 of the article cited in reference 44. t This problem of ring closure of allylphenols is of i*iportanoe in the chemistry of vitamin E and has been discussed in detail by Smith (reference 78). '• Schopf and co-workers, Ann., 044, 30 (1940). »• Mauthner, / . prakt. Chem., [2] 148, 95 (1937). " Hahn and Stenner, Z. physM. Chem., 181, 88 (1929). 78 Smith, Chem. fieiia., 27, 287 (1940)....
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