Aldehydes P,y-unsaturated

If an OR or SR group is attached to the negative carbon, the reaction becomes a method for the preparation of p,y-unsaturated aldehydes, because the product is easily hydrolyzed. 

We describe here (i) the selective hydroformylation of 1,3-dienes to p,y-unsaturated aldehydes promoted by catalyst A and (ii) the efficient hydrosilylation of aromatic nitriles to A,iV-disilylamines using catalysts B and C. 

To direct a solvolytic ring opening, 2-methoxycyclopropyllithium (6) was developed as a chain extension conjunctive reagent. The failure of P-elimination to occur in 6 presumably derives from the high strain of cyclopropene and poor orbital overlap for elimination. The aldehyde adducts smoothly solvolyze to give p,Y unsaturated aldehydes (Eq. 22) 23) which are best initially isolated as their hemithioacetals. 

Irradiation of the cycloalkenone (218) with vinylene carbonate in THF resulted in the separation of (219), which yielded the p,y-unsaturated aldehyde (220) upon treatment with KOH in MeOH at room temperature (Table 12) 73 . 

The above-described results have changed the ideas that most photochemists had about the photoreactivity of p,y-unsaturated aldehydes. From the earlier work carried out mainly by Schaffner, Diirr, and Pratt [4] on a series of compounds of this type, a general consensus surrounding their lack of ODPM reactivity was developed [17]. Most of the compounds studied previously undergo decarbonyla-... 

Ring-closing metathesis seems particularly well suited to be combined with Passerini and Ugi reactions, due to the low reactivity of the needed additional olefin functions, which avoid any interference with the MCR reaction. However, some limitations are present. First of all, it is not easy to embed diversity into the two olefinic components, because this leads in most cases to chiral substrates whose obtainment in enantiomerically pure form may not be trivial. Second, some unsaturated substrates, such as enamines, acrolein and p,y-unsaturated aldehydes cannot be used as component for the IMCR, whereas a,p-unsaturated amides are not ideal for RCM processes. Finally, the introduction of the double bond into the isocyanide component is possible only if 9-membered or larger rings are to be synthesized (see below). The smallest ring that has been synthesized to date is the 6-membered one represented by dihydropyridones 167, obtained starting with allylamine and bute-noic acid [133] (Fig. 33). Note that, for the reasons explained earlier, compounds... 

Additions of allylic boron reagents have been reported on a very wide range of classes of functionalized aldehydes. Some types of aldehydes, however, are very reactive and may lead to side-reactions. For example, p,Y-unsaturated aldehydes are notoriously difficult substrates but an indirect procedure for their in situ generation leads to clean products of allylboration. Although most examples... 

RuCl/TMP), as RuCl2(TMP)/(Cl2pyNO)/CDCl3, oxidised alkenes RCHj=CH to the aldehydes RCH CHO. Oxidation of 1,3-dienes gave the unsaturated aldehyde -thus 1-phenyl-l,3-butadiene gave the p, y-unsaturated aldehyde 4-phenylbut-3-enal (styrylacetaldehyde) RuClj(TDCPP) also effected these reactions [584],... 

Asymmetric hydroformylation of conjugated dienes has been almost unexplored. Recently, however, promising results were reported when Rh(acac)(CO)2/(E,5)-BINAPHOS (6) is used as the catalyst. Reactions give the corresponding p,y-unsaturated aldehydes with high enan-tiopurity (<96% ee) and regioselectivity (78-95% branched) (Scheme 7.5) [20,81],... 

We knew that a-alkoxy-P,y-unsaturated aldehydes were not suitable for Evans condensations... 

We could prepare a-alkoxy-y-phenylselenenyl aldehydes as "stable" equivalents of a-alkoxy-P,y-unsaturated aldehydes but their reaction with boron enolates did not lead to the expected condensation products, probably due to inadequate work up conditions. 

Maximally unsaturated azepines feature during the period under review. Cyclisation of the P,y-unsaturated aldehyde (1) by treatment with trifluoroacetic acid, followed by treatment with base (DABCO or DMAP) affords 2//-azepine <95AG(E)I469>. The compound itself is difficult to isolate but it is remarkably stable in solution, no trace of the thermodynamically favoured 3/f-azepine being observed after 48h at room temperature. In addition, the bicyclic adduct (2) is formed, probably arising from [2 + 4] cycloaddition of isobutene formed in situ and the intermediate immonium ion (3). 

Steric and electronic effects on the photochemical reactivity of oxime acetates of p/y-unsaturated aldehydes. Journal of the Chemical Society, Perkin Transactions 1, 163-169 (b) Armesto, D., Horspool, W.M., Mancheno, M.J., and Ortiz, M.J. (1990) The aza-di-jt-methane rearrangement of stable derivatives of 2,2-dimethyl-4,4-diphenylbut-3-enal. Journal of the Chemical Society, Perkin Transactions... 

As can be expected, use of ethyl diazoacetate procides y-oxoesters15) ory-oxocarboxylic acids 16) from enol ethers. Emploging the Julia method with 25 leads to the p,y-unsaturated aldehyde 26. Thus, this sequence establishes an overall a-vinylation of a given aldehyde n 17 18). 

Homologation of ketones to aldehydes cf. 9, 62-163), The reagent is particularly useful for conversion of an ot,p-unsaturated ketom, into the homologated p,y-unsaturated aldehyde (equation 1),... 

Unsaturated acetals. Reaction of ethyl diazoacetate with dimethyl acetals of a,(3-unsaturated aldehydes catalyzed by BE, ethciate gives as the main product acetals of P. y-unsaturated aldehydes by a carbon-carbon insertion. A similar reaction with ketals gives a complex mixture. The reaction is less selective with diethyl acetals. 

A concluding result, albeit of little synthetic value, should still be mentioned because of its uniqueness. One congener (41) of the otherwise ODPM-resistant p.y-unsaturated aldehyde class exhibits ODPM reactivity on direct excitation, giving rise to the formation of (42), albeit in moderate yield, besides decar-bonylation and 1,3-acyl migration (- 43) as the major reactions (Scheme 8). ... 

Especially interesting is the possibility for asymmetric synthesis by employing amino acid derived ylide systems e.g. 33 Scheme 54), which allows the synthesis of optically active p, y-unsaturated aldehydes. ... 

That isomerization of the p, y-unsaturated aldehyde to the conjugated isomer during hydrolysis is negligible as in the case of (158 159a equation 47) is no standard feature of this reaction. Presumably, smaller strain of the deconjugated isomer contributes to leaving certain of these aldehydes unaltered. Mostly due to this hindrance of isomerization, the carboxylation (160) - (161) could be accomplished with excellent stereocontrol (equation 48). ... 

Reaction of crotylchromium with a-methyl-p.y-unsaturated aldehyde (87) afforded (88) as the major diastereomer. 2 Thg other Cram product (89), which is expected to arise from an antiperiplanar transition state (46 Scheme 3), is obtained from a BFs-catalyzed tributylcrotylstannane addition. The remaining members of the stereo triad can be accessed by inversion of the C-2 hydroxy (i.e. 88 to 91 and 89 to 90)... 

Julia, in a very elegant approach, has used sulfur ylidic 3,2-rearrangements to produce p.y-unsaturated aldehydes (Scheme 46). Following the same idea, Kociensky also used ylidic 3,2-sigmatropic rearrangements to prepare P,7-unsaturated aldehydes he employed sulfur ylides in conjunction with silicon substituents (Scheme 47). In contrast, Mander used the nitrogen ylidic 3,2-rearrangement as an easy route towards the synthesis of P,y-unsaturated aldehydes (Scheme 48). ° It is noteworthy that better... 

A conceptually different entry into the preparation of dienolates and their use in vinylogous Mannich reactions was developed by Lautens and coworkers. Vinylox iranes can effectively be ring opened with catalytic amounts of a Lewis acid to furnish P,y unsaturated aldehydes that are in equilibrium with their dienol tautomer. As such this transformation involved an umpolung of the formerly electrophilic vinyl epoxide to the nucleophilic dienol that can be treated with electrophiles. 

Cookson has studied substrates that can, in principle, effect a tandem Claisen-Cope-Cope rearrangement. By employing 1,1,3-trimethoxybutane as a surrogate for 1-methoxy-1,3-butadiene (equation 10), allylic alcohol (62) undergoes acid-catalyzed exchange to provide the transient p,y-unsaturated aldehyde (66) which, lacking a substituent at the a-position, suffers a facile conjugation of the double bond that preempts the tandem process. This phenomenon has been also observed by Thomas. ... 

While hydroformylation of 1,3-butadiene has been intensively studied for the purpose of obtaining adipic acid [84,85], asymmetric hydroformylation of conjugated dienes has remained almost unexplored. A highly selective asymmetric hydroformylation of 1,3-dienes was reported using Rh(acac)[(i ,S)-BINAPHOS] as catalyst to give optically active p,y-unsaturated aldehydes (Scheme 9) [86,87]. 

A derivative of (S)-prolinol 10.23, quartemized by CICH2CN and then transformed into the ammonium ylide by KO-fert-Bu in DMSO, has been subjected to a [2,3]-sigmatropic rearrangement at -90°C. After hydrolysis of the aminoni-trile formed in this way, an a-chiral P,y-unsaturated aldehyde is formed with an excellent selectivity [261,290, 1008, 1062] (Figure 10.9). 

P,y-Unsaturated aldehydes These reagents serve as the equivalent to the unknown Wittig reagent (CgHs )3P=CHCH2CHO. Thus aldehydes react with... 

It is surprising that these two highly inefficient reactions are the only routes followed. It is to be noted, however, that triplet-induced 1,2-acyl shifts are only efficient when there is an enhanced n—ti band and su is not present in 114. The steroidal p,Y-unsaturated aldehyde 21 undergoes decarbonylation from the triplet state less efficiently than the 1,2- and 1,3-acyl shift. The related monocyclic aldehyde 120 only shows decarbonylation, and by analogy with 21 the reaction is suggested to stem from the triplet state. 3D... 

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