Synthesis via alkenes

Nawara AJ, Shima T, Hampel F et al (2006) Gyroscope-like molecules consisting of PdX2/ PtX2 rotators encased in three-spoke staUns synthesis via alkene metathesis, and facile substitution and demetalation. J Am Chem Soc 128 4962-4963... 

The reaction of o-iodophenol, norbornadiene and CO proceeds via alkene and CO insertions to afford the lactone 562, which is converted into coumarin (563) by the retro-Diels-Alder reaction. In this coumarin synthesis, norbona-diene behaves as a masked acetylene[4)3],... 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

As described in Section 2.3.2, vinylaziridines are versatile intermediates for the stereoselective synthesis of (E)-alkene dipeptide isosteres. One of the simplest methods for the synthesis of alkene isosteres such as 242 and 243 via aziridine derivatives of type 240 and 241 (Scheme 2.59) involves the use of chiral anti- and syn-amino alcohols 238 and 239, synthesizable in turn from various chiral amino aldehydes 237. However, when a chiral N-protected amino aldehyde derived from a natural ot-amino acid is treated with an organometallic reagent such as vinylmag-nesium bromide, a mixture of anti- and syn-amino alcohols 238 and 239 is always obtained. Highly stereoselective syntheses of either anti- or syn-amino alcohols 238 or 239, and hence 2,3-trans- or 2,3-as-3-alkyl-2-vinylaziridines 240 or 241, from readily available amino aldehydes 237 had thus hitherto been difficult. Ibuka and coworkers overcame this difficulty by developing an extremely useful epimerization of vinylaziridines. Palladium(0)-catalyzed reactions of 2,3-trons-2-vinylaziri-dines 240 afforded the thermodynamically more stable 2,3-cis isomers 241 predominantly over 240 (241 240 >94 6) through 7i-allylpalladium intermediates, in accordance with ab initio calculations [29]. This epimerization allowed a highly stereoselective synthesis of (E) -alkene dipeptide isosteres 243 with the desired L,L-... 

Crozet and co-workers have used S l reactions for synthesis of new heterocycles, which are expected to be biologically active (see also Section 7.3, which discusses synthesis of alkenes). For example, 2-chloromethyl-5-nitroimidazole reacts with the anion of 2-nitropropane to give 2-isopropylidene-5-nitroimidazole. It is formed via C-alkylation of the nitronate ion followed by elimination of HN02 (Eq. 5.33).51a Other derivatives of nitroimidazoles are also good substrates for SrnI reactions.5113 0... 

Stereoselective Alkene Synthesis via 1-Chloro-1-[(dimethyl)-phenylsilyl]-alkanes and a-(Dimethyl)phenylsilyl Ketones. 

The alcoholate is either dehydrogenated via (3-CH activation to afford an M-OH function and a hydrido (olefin) metal species or, to a minor extent, converted to alcohol. The hydrido (olefin) metal species can release 1-alkenes. The route toward alcohol is regarded as reversible, and thus allows the incorporation of co-fed alcohol, causing initiation of the FT synthesis via the alcoholate and the C,O-bridging aldehyde... 

Stereocenters were introduced in the synthesis via asynunetric epoxidation of triene-yne 40. Only two of the three more electron-rich alkenes were selectively epoxidized, leaving the alkene closest to the sulfone group unreacted. 

C-Glycoside synthesis via the intermolecular addition of anomeric radicals to activated alkenes has been pioneered by Giese and co-workers [120]. A wide variety of acceptors have been used in this area and both simple [131,132] and more complex (a C-disaccharide 290) [133] examples are shown (Scheme 77). 

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