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Hydroboration with thexylborane

Thexylborane adds to hindered olefins only once, and the product hydroborates only unhindered olefins thus leading to mixed trialkylboranes. This selective stepwise hydroboration with thexylboranes gives a simple synthesis of unsymmetrical ketones after carbonylation (Eqs. 19 and 20)41-58,59). [Pg.37]

Hydroboration with Thexylborane and other Alkyl- and Aryl-boranes Hydroboration with Monohaloboranes... [Pg.703]

The facial selectivity of (43) was found to be 1.75 1 from the ratio of aldol products (48) and (49) obtained by the reaction with the achiral boron enolate (50). The latter is structurally similar to reagents (37) and (38). These experiments confirm again the validity of the rule of double asymmetric synthesis. The product (44) can be further converted through a sequence of reactions to provide (+)-Prelog-Djerassi lactonic acid (51 Scheme 27) (i) trimethylsilylation (ii) hydroboration with thexylborane (single asym-... [Pg.251]

A convenient annulation procedure based on cycHc hydroboration—carbonylation of 1-vinyl- and l-aHylcycloalkenes with thexylborane provides trans-fused bicychc ketones (312). [Pg.317]

High levels of asymmetric induction have been achieved in the hydroboration of 1,3-, 1,4-, and 1,5-dienes with thexylborane (482,483,489,490). The first chiral center is formed by an intermolecular reaction. In the second step, the organoborane intermediate undergoes an intramolecular hydroboration, creating the second chiral center with high diastereoselectivity. [Pg.322]

Stereoselective cyclic hydroboration. The hydroboration of l,4 and 1,5-dienes with thexylborane generally results in boracycles.2 If the intial hydroboration produces an asymmetric center, the second intramolecular hydroboration can be effected with remote asymmetric induction. Thus hydroboration of the diene 1 with thexylborane (1.25 equivalent) followed by oxidation results in a 6 1 mixture of the diastereomers 2 and. 3. An even higher induction is obtained in the case of the 1,4-... [Pg.397]

Acyclic diastereoselective hydroboration,J Hydroboration-oxidation of terminal alkenes substituted at C4 by a large and a medium alkyl group can proceed asymmetrically with BH3 S(CH,)2 or, even more selectively, with thexylborane. Example ... [Pg.601]

Hydroboration-reduction of enones.2 Hydride reduction of a carbonyl group can be used to induce asymmetric intramolecular hydroboration of a double bond via a cyclic transition state. Thus reaction of the enone 1 with thexylborane (1 equiv.) followed by oxidation provides the 1,5-diol 2 with high 1,4-syn selectivity. A similar reaction with the homologous enone provides a 1,6-diol with modest 1.5-syn selectivity (syn anti = 6.6 1). [Pg.327]

Cyclic hydroboration of ( + )-limonene with thexylborane yields the pure borabicyclic compound (138), which is oxidized to the diol (32 X = OH) or protonol-ysed with acetic acid to yield pure (—)-carvomenthol (32 X = H) after the usual oxidation.227 The treatment of dialkylcyanothexylborates with trifluoroacetic anhydride followed by oxidation as a route to ketones has been used with ( + )-limonene to give the two bridged-ring ketones (139).228... [Pg.30]

More remote stereoselcction is achieved in the hydroboration of 1,4- and 1,5-dienes with thexylborane. The first stereogenic center is created by intermolecular hydroboration of the terminal double bond. This then directs the intramolecular addition to the second double bond proceeding via a cyclic transition state and leading to high levels of selectivity46. [Pg.27]

The reactions of hindered alkenes with thexylborane are generally accompanied by a significant amount of dehydroboration to give 2,3-dimethyl-2-butene. Thus, a very large excess of the latter is needed in order to produce (monomeric) dithexylborane. Hydroboration of a-pinene (1 1 ratio) with thexylborane results in ca. 15% dehydroboration.However, alkenes of somewhat lower steric requirements, such as simple 1,2-disubstituted ethylenes, can be converted fairly cleanly into the corresponding thexylmonoalkylboranes e.g. equation 13). These are mixed dialkylboranes and behave like other fairly hindered dialkylboranes (Section 3.10.4.3). They can hydroborate alkenes of lower steric requirements to give totally mixed trialkylboranes. [Pg.709]

Azacyclanones.3 The usual syntheses of medium-sized azacyclanones involve Dieckmann or acyloin cyclization conducted under high dilution. An interesting new approach involves hydroboration-cyanidation (4, 446-447 5, 606-607) of a diunsaturated carbamate such as 1. Hydroboration of 1 with thexylborane followed by cyanidation under standard conditions gives the cyclic ketone 2 in moderate yield in one step. The product can be reductively cyclized to the indolizidine alkaloid <5-coniceine (3). [Pg.203]

Two equivalents 1-alkyne with thexylborane produces the thexyldialkenylboranes . Thexylalkylalkenylboranes can be obtained either by sequential hydroboration of an alkene-alkyne pair with thexylborane [Eq. (ct)] or via hydroboration of an alkene with thexylchloroborane, followed by reduction and hydroboration of an alkyne [Eq. (cw)]. Cyclic trialkylboranes are synthesized via hydroboration. Thus, HjB-THF reacts 2 3 with simple acyclic dienes to give predominantly the dumbbell-shaped organoboranes. When heated, they undergo isomerization to the more thermodynamically stable isomers. These empirical rules predict their thermal behavior (a) the thermal... [Pg.122]

Scheme 1). Preparation of 1,3-diols by hydroboration of allylic alcohols with thexylborane has been shown to proceed with high stereoselectivity when the substrates are 1-(1-hydroxyalkyl)-cyclohexenes. Stereocontrol in the hydrosilation of allylic and homoaHylic alcohols has been studied jointly by the groups of Tamao... [Pg.208]

Unsymmetrical cyclic ketones can be formed by the hydroboration of dienes followed by the oxidation sequence just described. In a typical example, Bryson reacted 176 with thexylborane but sodium cyanide was present (in situ) and this mixture generated 177. When this product was treated with trifluoroacetic anhydride... [Pg.478]

Syntheses of /(-(-i-)-myoporone (em-12) and / -(-H)-10,l 1-dehydromyoporone (ent-14) have been published recently (22). These rely on the use of methyl 5-citronellate as the source of the asymmetric carbon (Scheme 3). Of more general interest and application is an approach (23) which leads to the stereospecific synthesis of 9-dihydromyoporone, which contains two asymmetric centres. This synthesis formally constitutes a synthesis of myoporone. Still and Darst (23) have shown that hydroboration of -2,6-dimethylhepta-1,4-diene with thexylborane leads, with high diastereoselectivity, to the diastereomeric 1,4-diols (Scheme 4). Appropriate transformation of the yn-diol to the electrophilic iodo compound, followed by alkylation of the P-keto sulphoxide, gives... [Pg.229]

Methoxyoestra-l,3,5(10)-triene (122) has been prepared stereospecihcally by a hydroboration-carbonylation procedure. Treatment of the diene (120) with thexylborane resulted in regio- and stereo-selective formation of the carbaborane (121). Carbonylation and oxidation of (121) led to the tricyclic ketone (123) which was readily cyclized by acid catalysis to (122). [Pg.220]

Carbonyl compounds are an important class among organic molecules. Literature records several methods for their synthesis. However, there are very few methods to convert carbon-carbon unsaturation to carbonyl compounds. Hydroboration of acetylenes, followed by oxidation provides a novel method for carbonyl synthesis. It has been noted that regioselectivities achieved in the monohydroboration of internal acetylenes with thexylborane [1], disiamylbo-rane [1], dicyclohexylborane [1], and catecholborane [2] are similar to, but less pronounced than, that realized by 9-BBN [3]. The B-alkenyl-9-BBN derivatives undergo oxidation to the corresponding ketones or aldehydes under aprotic conditions with trimethylamine N-oxide [4, 5] or under protic conditions by inverse addition to buffered hydrogen peroxide [3]. The inverse addition, i.e., the slow addition of the B-alkenyl-9-BBN in THF to the buffered H O, suppresses the otherwise undesirable protonolysis reaction and favors the oxidation pathway to the desired aldehyde or ketone. [Pg.213]

Hydroboration. Metallacycloalkanones have been prepared by the cyclic hydroboration of dialkenyl silanes. In order to obtain one such dialkenyl silane, chlorodimethylvinylsilane was allylated with allyl magnesium bromide (eq 4).3 The dialkenyl silane 6 was then hydroborated sequentially with thexylborane, followed by treatment with potassium cyanide, trifluoroacetic anhydride, and finally hydrogen peroxide in sodium hydroxide to produce the silacycloheptanone 7 (eq 4). Cyclic silylketones were then further reacted to form acyclic compounds with chiral centers that are difficult to introduce without the use of the silicon tether. [Pg.158]

Brown, H.C. and Negishi, E. (1967) Carbonylation of the organoboranes from the cyclic hydroboration of dienes with thexylborane. Simple procedure for the conversion of dienes into cyclic ketones. Journal of the American Chemical Society, 89,5A11-5A7S. [Pg.373]


See other pages where Hydroboration with thexylborane is mentioned: [Pg.76]    [Pg.17]    [Pg.247]    [Pg.40]    [Pg.76]    [Pg.17]    [Pg.247]    [Pg.40]    [Pg.48]    [Pg.1044]    [Pg.77]    [Pg.38]    [Pg.4]    [Pg.121]    [Pg.38]    [Pg.461]    [Pg.318]    [Pg.52]    [Pg.34]    [Pg.144]    [Pg.387]    [Pg.365]    [Pg.25]    [Pg.35]   
See also in sourсe #XX -- [ Pg.2 , Pg.251 ]

See also in sourсe #XX -- [ Pg.251 ]

See also in sourсe #XX -- [ Pg.251 ]

See also in sourсe #XX -- [ Pg.2 , Pg.251 ]

See also in sourсe #XX -- [ Pg.251 ]




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Hydroboration thexylborane

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