Ketones, a-bromo-p-hydroxy

Homologation of ECHO to a-bromo-a, -enones.4 The initial steps of this homologation involve addition of dibromomethyllithium (1) to an aldehyde followed by oxidation of the adduct to a dibromomethyl ketone (2). The aluminum enolate of 2 undergoes an aldol reaction to provide an a-bromo-p-hydroxy ketone... 

Tin(n) triflate mediated cross aldol reactions between a-bromo ketone (124 Scheme 56) and aldehydes afford iyn-a-bromo-P-hydroxy ketones (125) with high stereoselectivity. The resulting halohydrins are converted to the corresponding (Z)-2,3-epoxy ketones (126). Chiral aldehyde (127) reacts with lithium alkynide (128) followed by mesylation and base treatment to give chirally pure ( )-epoxide (129). The initially formed alkoxide anion should be trapped in situ by mesylation, otherwise partial racemization takes place owing to benzoate scrambling (Scheme 56). ... 

P. V. Efficient diastereoselective synthesis of anti-a-bromo /1-hydroxy ketones. Tetrahedron Lett. 1999, 40, 7875-7877. 

The Darzens reaction has been performed enantioselectively, by coupling optically active a-bromo-p-hydroxy esters with aldehydes.Chiral phase-transfer agents have been used to give epoxy ketones with modest enantioselectivity. Chiral additives have proven to be effective. 

P-Hydroxy ketones. a-Bromo ketones undergo a Reformatsky-type reaction with aldehydes which is promoted by R3Sb but which is only possible with iodine as catalyst. The function of I2 is probably for a Br/I exchange, since I2 is not required in reactions with a-iodo ketones. 

Bromomethyl ketones. Aldehydes can be converted into bromomethyl ketones by reaction with the anion of bromomethyl phenyl sulfoxide to form a p-hydroxy-a-bromo sulfoxide followed by pyrolysis in xylene or diglyme (equation I). ... 

Treatment of the epoxy sulfones obtained thus with MgBr2 produces a-bromo ketones or a-bromo al-dehyde. This conversion can be applied to stereoselective preparations of a-bromo methyl ketones from cyclohexanone derivatives. Thus, treatment of 17-P-hydroxy-5-a-androstan-3-one (134) with 1-chloroethyl phenyl sulfone provides an epoxy sulfone (135), which is cleaved by MgBr2 to give a 99 1 mixture of 3-acetyl-3-bromoandrostanes (136 and 137 equation 33), whereas similar treatment of 17-p-hydroxy-5-B-androstan-3-one (138) gives a 4 96 mixture of 3-acetyl-3-bromoandrostanes (139 and 140 equation 34). These facts may reflect the steric course of the initial attack of the a-sulfonyl carbanion on the carbonyl face. 

Other procedures for the synthesis of P-ketophosphonates based on the use of a phosphorus electrophile have been explored. For example, the reaction of dialkyl chlorophosphates with the dilithium derivatives of a-bromo ketones has been described,but the yields of isolated products remain modest. The Lewis acid-catalyzed reaction of oc-hydroxy ketones with dialkyl chlorophosphites constitutes an efficient synthesis of P-ketophosphonates in high yields (92-96%, Scheme 7.42). This reaction is of special value for the preparation of oc-fully substituted P-keto-phosphonates. ... 

Cerium enolates may also be generated reductively in THF from a-iodo or a-bromo esters in the presence of an aldehyde or ketone using cerium metal turnings and catalytic HgCh (or cerium amalgam).35 This leads to a Reformatsky-type reaction (Volume 2, Chapter 1.7) producing the p-hydroxy ester (47) as a mixture of diastereomers (equation 16). 

Aldol reactions. Aldol products are obtained in good yields from reaction of ketones with glyoxylic acid monohydrate with assistance of ultrasound irradiation. Substrate-control (by 1,3- + 1,5-asynmietric induction) of the aldol reaction involving y-amino-a-ketoesters under solvent-free conditions is very effective.- With lithium dicyclohexylamide and InCl, the reaction of esters with aldehydes furnishes P-hydroxy esters, and that of a-bromo esters affords a,p-epoxy esters." These are typical Reformatsky and Darzens reaction products, respectively. 

Willow-like cascade construction was facilitated by the preparation of the AB2-type building blocks, e.g., 6-bromo-l-(4-hydroxy-4 -biphenylyl)-2-(4-hydroxyphenyl)hexane (40 Scheme 6.12), 13-bromo-l-(4-hydroxyphenyl)-2-[4-(6-hydroxy-2-naphthalenylyl)-phenyljtridecane (41), and 13-bromo-l-(4-hydroxyphenyl)-2-(4-hydroxy-4"-p-terpheny-lyl)tridecane (42). Scheme 6.12 illustrates the monomer preparative strategy for the incorporation of the monomeric mesogenic moieties predicated on conformational (gauche versus anti) isomerism. Thus, one equivalent of 4-hydroxybiphenyl was allowed to react with 1,4-dibromobutane to yield the monobromide 43, which was converted under Finkelstein conditions to the corresponding iodide 44. Treatment of the latter with ketone 45 under phase transfer conditions afforded the a-substituted ketone 46. Reduc-... 

DIELS-ALDER DIENOPHILES Bis(2,2,2-trichloroethyl) azodicarboxylate. 1-Bromo-2-chlorocyclopropene. Chromium carbene complexes. (R)-Ethyl p-tolysulfinyl-methylenepropionate. (S)-a-Hydroxy- 3, 0-dimethylpropyl vinyl ketone. (lR)-c/s-3-Hydroxy isobomyl neopentyl ether. Oxa-zolidones, chiral. Phenylselenenyl ben-zenesulfonate. 

A curious reaction occurred when a ib-bromo-iy-ketone (13) was treated with sodium methoxide [47]. The Favorsldi rearrangement is impossible in this structure (p. 206), and instead the compound reacted to give the iba-hydroxy-iy.iy-... 

Reaction of 16-bromo-17-ketone with hydrazinei6a-hydroxy-i7 hydrazone P. Catsoulacos and A. Hassner, J. Org. Chem., 32 (1967)... 

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