Methyl migration with dehydration

J -3-ketosteroids 17, 824 4-organothiomethyl-d -3-ketosteroids 17, 824 Methyl migration with dehydration 14, 942 1,3-C-Methyl —, photochemical 16, 755... 

In the dehydration of 3,3-dimethyl-2-butanol, a secondary carbocation is formed initially, which rearranges to a tertiary carbocation when a neighboring methyl group with its bonding electron pair migrates to the positive carbon. The charge is thereby transferred to the tertiary carbon ... 

B-8. Which of the following alcohols would be most likely to undergo dehydration with rearrangement by a process involving a methyl migration (methyl shift) ... 

Westphalen-Lettre rearrangement. Dehydration of 5-hydroxycholesterol derivatives accompanied by C-10 to C-5 methyl migration in compounds with a (i-SLibstitucnt in C-6. 

Along with the desired product, dehydration under the reaction conditions provided 20 and 21. Methyl migration from the formed methyl carbocation 24 provides the second major product 19, which also produces dehydration products 22 and 23. 

Medroxyprogesterone acetate (74) is stmcturaHy related to and has been prepared from hydroxyprogesterone (39) (Fig. 10). Formation of the bis-ketal accomplishes the protection of the ketones and the required migration of the double bond. Epoxidation with peracetic acid produces a mixture of epoxides (75), with a predominating. Treatment of the a-epoxide with methyl magnesium bromide results in diaxial opening of the epoxide. Deprotection of the ketones provides (76), which is dehydrated to (77) by treatment with dilute sodium hydroxide in pyridine. Upon treatment with gaseous hydrochloric... 

Without exception the 17-secondary alcohols give three products, the dehydration products with migration of the 13j5-methyl group, the 17a-fluoro derivatives and the 17/5-chlorofluoroacetates. ... 

The dehydration of neopentyl alcohol can best be explained by a concerted mechanism involving the removal of the proton from the y-carbon atom by the basic sites and of the hydroxyl group by the acid sites of the alumina, with migration of the methyl group ... 

Meerwein rearrangements, a i7a-hydroxyandrostane (21) is apparently dehydrated easily with migration of the Cps) methyl group from C(i3> to Cd ) [124]. The stereoelectronic requirements in the I7 3 epimer (22) would be satisfied by migration of the C(i4)- C(ig) bond to C(i4>"-C(i7), but the contraction of ring D to a highly strained tmns fused cyclobutane structure must be less favourable than the observed cis ... 

The immediate product from migration of the angular methyl group to Cp ) is a Cps)-carbonium ion (25), the fate of which varies according to the compound. Solvolysis of a secondary i7j -tosylate, or acid-catalysed dehydration of the i7j3 alcohol, leads to a mixture of olefins containing the isomer 26) as the major product, with minor amounts of j i3(i4) 27) and other olefins. An elaborate analysis by Johns [126] of the olefins obtained by the boric add-catalysed dehydration of oestradiol 3-methyl ether at high temperatures led to the characterisation of at least six isomeric olefins in the mixture. The precise mechanism of the action of boric acid has not been defined. It may involve thermal decomposition of a borate ester or merely the expulsion of the protonated... 

The pinacol rearrangement was first described by Fittig in 1860 [10]-treatment of pinacol (2,3-dimethyl-2,3-butanediol, 1), a ditertiary 1,2-diol, with sulfuric acid resulted in the formation of pinacolone (3,3-dimethyl-2-butanone, 2) (Scheme 1). The overall reaction is dehydration with concomitant migration of a methyl substituent. 

A new vitamin A synthesis has been described featuring an efficient dehydration step without double bond migration Olefins can be obtained from glycols with complete control of the position as well as the stereochemistry of the double bond. Together with Irans-hydroxylations this reaction constitutes a general and unambiguous method for the interconversion of ci5- and trans-olefins Monoperoxysuccinic acid has recently been recommended for the conversion of olefins into trans-glycols Also a convenient preparation of methyl ketones from terminal olefins has been reported... 

Sulfuric acid Dehydration of alcohols with conjugation and methyl group migration... 

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