Selective and Partial Reductions

Suggest reagents and reaction conditions that would be suitable for each of the following selective or partial reductions ... 

Aldehydes are prepared by the hydroboration-oxidation of alkynes (see Section 5.3.1) or selective oxidation of primary alcohols (see Section 5.7.9), and partial reduction of acid chlorides (see Section 5.7.21) and esters (see Section 5.7.22) or nitriles (see Section 5.7.23) with lithium tri-terr-butox-yaluminium hydride [LiAlH(0- Bu)3] and diisobutylaluminium hydride (DIBAH), respectively. 

The selective or partial reduction of dihalocyclopropanes to monohalocyclopropanes107 can be achieved with LAH108 or Bu3SnH (equations 32 to 34).109,11° A number of other reducing agents also lead to partial reduction. Reduction of dihalocyclopropanes proceeds via radical intermediates and the cyclopropyl radical thus generated can be trapped by an adjacent homoallylic double bond leading to a bicyclo[3.1.0]hexane system (equation 35).111,112... 

The reactions of heterocyclic compounds composed of fused systems (6 6 6), such as electrophilic and nucleophilic attack at a ring or a substituent and partial reduction, are often highly selective. Unfortunately, there are few unifying features among the reactivities of the three different classes of systems, and a detailed treatment of the reactivity is beyond the scope of this concise chapter. All references to the chemistry of a particular parent compound and derivatives are given in Tables U3. 

Sec. alcohols from ketones Partial, selective, and total reduction... 

Partial, selective, and stereospecific reduction of ketones to sec. alcohols. 

It has been found that at 293K the short illumination of Ryiridis RCs by continuous light at low redox potential (prereduced quinones and cytochrome) causes the reduction of HL and partially HM [19] (Fig.IB). Further illumination of RCs at 77K is accompanied by the reduction of HM and partial reduction of BL (or BM) since the bleachings of HM and BL (BM) bands and the developing the characteristic band of radical anion of BChl b at 1060 nm [23] are observed. The illumination of R.viridis RCs frozen in the dark at low redox potential down to 77K induces the selective reduction of HL (Fig.lA) [19]. 

With the co side chain at C-12 in place, we are now in a position to address the elaboration of the side chain appended to C-8 and the completion of the syntheses. Treatment of lactone 19 with di-isobutylaluminum hydride (Dibal-H) accomplishes partial reduction of the C-6 lactone carbonyl and provides lactol 4. Wittig condensation8 of 4 with nonstabilized phosphorous ylide 5 proceeds smoothly and stereoselectively to give intermediate 20, the bistetra-hydropyranyl ether of ( )-1, in a yield of -80% from 18. The convergent coupling of compounds 4 and 5 is attended by the completely selective formation of the desired cis C5-C6 olefin. 

TV-aluminum imines are another example of masked inline derivatives of ammonia. They are easily synthesized by partial reduction of nitriles with diisobutylaluminum hydride (D1BAL-H)6. Addition of lithium organic reagents to /V-aluminum iniines 7 derived from O-protected cyanohydrins 6 provides a-amino alcohols 8a and 8b in moderate yields and low to good diastereo-selectivities n 12. 

An obvious drawback in RCM-based synthesis of unsaturated macrocyclic natural compounds is the lack of control over the newly formed double bond. The products formed are usually obtained as mixture of ( /Z)-isomers with the (E)-isomer dominating in most cases. The best solution for this problem might be a sequence of RCAM followed by (E)- or (Z)-selective partial reduction. Until now, alkyne metathesis has remained in the shadow of alkene-based metathesis reactions. One of the reasons maybe the lack of commercially available catalysts for this type of reaction. When alkyne metathesis as a new synthetic tool was reviewed in early 1999 [184], there existed only a single report disclosed by Fiirstner s laboratory [185] on the RCAM-based conversion of functionalized diynes to triple-bonded 12- to 28-membered macrocycles with the concomitant expulsion of 2-butyne (cf Fig. 3a). These reactions were catalyzed by Schrock s tungsten-carbyne complex G. Since then, Furstner and coworkers have achieved a series of natural product syntheses, which seem to establish RCAM followed by partial reduction to (Z)- or (E)-cycloalkenes as a useful macrocyclization alternative to RCM. As work up to early 2000, including the development of alternative alkyne metathesis catalysts, is competently covered in Fiirstner s excellent review [2a], we will concentrate here only on the most recent natural product syntheses, which were all achieved by Fiirstner s team. 

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

In synthesis, the principal factors that affect the choice of a reducing agent are selectivity among functional groups (chemoselectivity) and stereoselectivity. Chemo-selectivity can involve two issues. One may wish to effect a partial reduction of a particular functional group or it may be necessary to reduce one group in preference to another,78 In the sections that follow, we consider some synthetically useful partial and selective reductions. 

The most widely used reagent for partial reduction of esters and lactones at the present time is diisobutylaluminum hydride (DiBAlH).83 By use of a controlled amount of the reagent at low temperature, partial reduction can be reliably achieved. The selectivity results from the relative stability of the hemiacetal intermediate that is formed. The aldehyde is not liberated until the hydrolytic workup and is therefore not... 

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