Regioselective hydrogenations dienes

The reaction has been successfully applied to the synthesis of the precursor 366 of provitamin D which has the homoannular conjugated diene in the B ring [169]. Treatment of 7a-carbonate 364 with the Pd catalyst at 40 °C affords the 5,7-diene 366 regioselectively in good yield. No heteroannular diene 367 is detected. In the intermediate complex 365, the /f-oricnted 7(j-allylpalladium undergoes facile syn elimination of the 8 ft-hydrogen to afford 366 exclusively. 

A catalyst, usually acid, is required to promote chemoselective and regioselective reduction under mild conditions. A variety of organosilanes can be used, but triethylsilane ia the presence of trifiuoroacetic acid is the most frequendy reported. Use of this reagent enables reduction of alkenes to alkanes. Branched alkenes are reduced more readily than unbranched ones. Selective hydrogenation of branched dienes is also possible. 

The cycloaddition of photoenol of o-methylbenzaldehyde 66 with 5-alkyli-dene-l,3-dioxane-4,6-dione derivatives 67 is an example of a photo-induced Diels Alder reaction in which one component, the diene in this case, is generated by irradiation [48]. The yields of some cycloadducts 68, generated by photo-irradiation of a benzene solution of 66 and 67 at room temperature, are reported in Table 4.14. The first step of the reaction is the formation of (E)-enol 69 and (Z)-enol 70 (Equation 4.7) by an intramolecular hydrogen abstraction of 66 followed by a stereo- and regioselective cycloaddition with... 

In practice simple alkenes give poor cycloaddition so diene (49) was the chosen starting material and the aldol was protected and hydrogenated to give (48). Note that (50) is emphatically bowl shaped (Chapter T 34) so that substituents (OR) or reagents (Hg) prefer to be on the outside of the bowl. The regioselectivity of the addition of Cl2C=C=0 to the diene (49) is correct. Synthesis ... 

The event, oxidative addition of a Ni(0) species upon dienes and aldehydes activated by coordination with Lewis acids to provide oxanickellacycles 45, has proven to take place quite generally, and many variations making the best use of the intermediate 45 have been developed. The key issue of the reactions discussed in Sect. 3 is a regioselective and stereoselective hydrogen delivery... 

Preparative electrochemical reduction of aryltrimethylsilanes in methyl-amine in the presence of LiCl gives the Birch-type products, 1,4-cyclohexan-dienes (Scheme 34) [6], A mechanism involving the electrochemical formation of lithium metal which chemically reduces the substrate has been suggested. The hydrogen atom is introduced on the carbon adjacent to the silicon preferentially. This regioselectivity is consistent with the spin density of the anion radical determined by ESR spectroscopy (Sect. 2.2.1). 

Another example, in which the piperidine cycle is generated de novo, exploits a hetero Diels-Alder cycloaddition of 1 -/r-tolylsulfinyl-1,3-penta-diene 91 with benzylnitrosoformate, that generates an oxazine 92 with complete regioselectivity and 7i-facial diastereoselectivity.69 Osmilation of the double bond inserts stereoselectively two hydroxyl groups on the oxazine skeleton, protection and catalytic hydrogenation finally afforded the enantiomerically pure imino sugars 94 (Fig. 38). 

An activation of the nucleophile by insertion of the transition metal species into a nucleophile-hydrogen bond. Here typical metals are Pd(0), Y(III), La(III) and Sm(III). Then one Jt-bond of the 1,2-diene in 13 inserts intramolecularly into the nucleophile-metal bond (Scheme 15.2). Depending on the regioselectivity of this insertion either 14, the possible equilibrium between 15,16,17 or 18 will be the next intermediates. Reductive elimination finally delivers 8 from 14, 10 and/or 11 from 15,16 or 17 and 12 from 18. 

By far most of the reports on addition reactions of hetero-nucleophiles to activated dienes deal with sulfur-nucleophiles17,48,80,120-137, in particular in the synthesis of 7/3-sulfur-substituted steroids which, like their carbon-substituted counterparts (Section n.A), are of interest because of their ability to inhibit the biosynthesis of estrogens80,129-137. Early investigations17,120-122 concentrated on simple acyclic Michael acceptors like methyl sorbate and 2,4-pentadienenitrile. Bravo and coworkers120 observed the formation of a 3 1 mixture of the 1,6- and 1,4-adduct in the reaction of methyl sorbate with methanethiol in basic medium (equation 39). In contrast to this, 2,4-pentadienenitrile adds various thiols regioselectively at C-5, i.e. in a 1,6-fashion (equation 40)17,121,122, and the same is true for reactions of this substrate with hydrogen sulfide (equation 41), sodium bisulfite and ethyl thioglycolate17. 

The hydrogenation of dienes to monoenes introduces several problems of selectivity. Regioselective saturation of one of the double bonds is governed basically by the same effects that determine the relative reactivities of monoalkenes in a binary mixture that is, a terminal double bond is reduced preferentially to other, more substituted double bonds. During the reduction of a diene, a new competition also emerges since the newly formed monoene and the unreacted diene compete for... 

The regioselectivity exhibited in the hydrogenation of monoenes also holds for the hydrogenation of dienes specifically, the terminal double bond in nonconju-gated dienes is usually reduced selectively 111... 

The difference in reactivity of different double bonds allows selective hydrogenation of dienes. Conjugated dienes with at least one trisubstituted double bond may be completely reduced by using excess reagents222 [Eq. (11.71)]. Regioselective hydrogenation of the more substituted double bond occurs in 1,5-dienes. The hydrogenation of 1,4-dienes, however, is not selective [Eq. (11.72)] ... 

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