Aldol sequential

Robinson Annulation Sequential Michael addition/aldol condensation between a ketone enolate and an alkyl vinyl ketone (i.e. MVK) to give a cyclohex-2-en-l-one... 

Robinson annulation reaction (Section 23.12) A synthesis of cyclohexenones by sequential Michael reaction and intramolecular aldol reaction. 

Figure 10.44 RibA catalyzed sequential aldol additions yielding a key chiral building block for cholesterol-lowering drugs.

A fourfold anionic sequence which is not initiated by a Michael but an aldol reaction has been reported by the group of Suginome and Ito (Scheme 2.129) [295]. In this approach, the borylallylsilane 2-573 reacts selectively in the presence of TiCl4 with two different aldehydes which are added sequentially to the reaction mixture. First, a Lewis acid-mediated allylation of the aldehyde with 2-573 takes place to form a homoallylic alcohol which reacts with the second aldehyde under formation of the oxenium ion 2-574. The sequence is terminated by a Prins-type cyclization of 2-574 and an intramolecular Friedel-Crafts alkylation of the intermediate 2-575 with formation of the fraws-1,2-be rizoxadeca lines 2-576 as single diastereomers. 

Vassilikogiannakis and coworkers described a simple sequential process for the biomimetic synthesis of litseaverticillol B (4-159) which includes a cycloaddition of 4-158 and singlet oxygen to give 4-160, followed by ring opening to afford the hydro-genperoxide 4-161 (Scheme 4.34) [55]. Reduction of 4-161 led to the hemiacetal 4-162, which underwent an aldol reaction to afford 4-159. 

Ono and coworkers have extended the radical elimination of v/c-dinitro compounds to P-nitro sulfones151 and P-nitro sulfides.138,152 As P-nitro sulfides are readily prepared by the Michael addition of thiols to nitroalkenes, radical elimination of P-nitrosulfides provides a useful method for olefin synthesis. For example, cyclohexanone is converted into allyl alcohol by the reaction shown in Eq. 7.110. Treatment of cyclohexanone with a mixture of nitromethane, PhSH, 35%-HCHO, TMG (0.1 equiv) in acetonitrile gives ahydroxymethylated-P-nitro sulfide in 68% yield, which is converted into the corresponding allyl alcohol in 86% yield by the reaction with Bu3SnH.138 Nitro-aldol and the Michael addition reactions take place sequentially to give the required P-nitro sulfides in one pot. 

Aldolases catalyze asymmetric aldol reactions via either Schiff base formation (type I aldolase) or activation by Zn2+ (type II aldolase) (Figure 1.16). The most common natural donors of aldoalses are dihydroxyacetone phosphate (DHAP), pyruvate/phosphoenolpyruvate (PEP), acetaldehyde and glycine (Figure 1.17) [71], When acetaldehyde is used as the donor, 2-deoxyribose-5-phosphate aldolases (DERAs) are able to catalyze a sequential aldol reaction to form 2,4-didexoyhexoses [72,73]. Aldolases have been used to synthesize a variety of carbohydrates and derivatives, such as azasugars, cyclitols and densely functionalized chiral linear or cyclic molecules [74,75]. 

Liu, J.J., Hsu, C.C. and Wong, C.-H. (2004) Sequential aldol condensation catalyzed by DERA mutant Ser238Asp and a formal total synthesis of atorvastatin. Tetrahedron Letters, 45, 2439-2441. 

Johnson has developed two linear approaches to synthesize the C-nor-D-homosteroid skeleton (Scheme 2.2). In his first approach [21], tetralone 19, obtained from reduction of 2,5-dimethoxynaphthalene, was used as the source of the C,D-rings. The B- and A-rings were constructed by sequential Robinson annulations (19 —> 20 —> 21). The Cl 1,12 olefin was then introduced to provide 22. Ozonolysis of 22 followed by an aldol reaction of the resulting dialdehyde gave 23. Subsequent deformylation and deoxygenation afforded the cyclopamine skeleton 24. 

Crossed aldol condensation of an anion generated a- to a ketone equivalent with o, /3-unsaturated aldehyde, dehydration and release of the ketone is an effective way of generation of dienones. Corey and Enders found that a-lithiated /V,/V-dirnethylhydrazones undergo 1,2-addition to the aldehydes and ketones to form /1-hydroxy derivatives. Sequential treatment of the intermediate with sodium periodate and methanesulphonyl chloride-triethylamine furnishes , -2,4-dienone derivative (equation 57)94. 

In Holton s and Wender s work, the total synthesis was achieved by sequentially forming the AB ring through the fragmentation of epoxy alcohols derived from (—)-camphor and a-pinene. Nicolau s, Danishefsky s, and Kuwa-jima s total syntheses involved B ring closure connecting the A and C rings, whereas in Mukaiyama s synthesis, the aldol reaction was extensively applied to construct the polycyclic system. 

With phenylhydroxylamine and an aldehyde, 1,2-propadienyl methyl ketone afforded indole derivatives via the intermediacy of 437 the latter was formed by a sequential addition, in situ aldol condensation and sigmatropic rearrangement process [195]. 

Zhang and Lu observed that the sequential reaction of an allenyl ketone, Bu4NI, ZrCl4 and an aldelyde in CH2C12 at -78 °C afforded 3-(l -hydroxyalkyl)-4-iodo-4-en-2-one 463 via conjugate addition and a subsequent aldol process [206]. 

General Procedure for Sequential Enolboration/Hydroformylation/Aldoladdition. Synthesis of Cyclic Aldol Products. NEt3 (1.05 eq to carbonyl compound) was precomplexed under an argon atmosphere with (cy-hex)2BCl (1.05 eq) in dry CH2C12 at 0 °C for 15 min. The unsaturated carbonyl compound was then added slowly via syringe and the enolbo-... 

The aryl aldehyde (1.1 mmol) and trimethylsilyl enol ether (1 mmol) are added sequentially to TBA-F (16 mg, 0.06 mmol) in THF (2 ml) at -78°C. The mixture is stirred at -78°C for 3-5 h, then warmed to room temperature, and H,0 (25 ml) is added. The aqueous mixture is extracted with Et,0 (3x15 ml) and the dried (MgS04) extracts are fractionally distilled to yield the aldol product (e.g. from PhCHO and 1-trimethylsilyl-oxycyclohexene, 84%, 6-methyl- 1-trimethylsilyloxycyclohexene, 68% 1 -trimethylsilyl-oxycycloheptene, 80%, 3-trimethylsilyloxypent-2-ene, 70%]. 

One-pot conversions of 2-hydroxy(acylbenzenes) with anhydrides or acid chlorides to produce coumarins [52-54] and flavones [54-58] under mild liquiddiquid or solidtliquid two-phase conditions via a Baker-Venkataraman type reaction (Scheme 6.19) are catalysed by quaternary ammonium salts. 3-Substituted coumarins are produced from salicylaldehyde and malonodinitrile, or substituted acetonitriles, in high yield (>85%) in a one-pot catalysed sequential aldol-type reaction and cycliza-tion in the absence of an added organic solvent [59]. When 2 -hydroxychalcones are reduced under catalytic two-phase conditions with sodium borohydride, 2,4-cis-flavan-4-ols are produced [60] (see Section 11.3). 

Scheme 1.57). Although the natural donor aldehyde is D-2-deoxyribose-5-phosphate, non-phosphorylated donor aldehydes are also tolerated and the enzyme displays some flexibility towards both donor and acceptor. Importantly, as both donor and acceptor substrates are aldehydes, the enzyme can perform sequential aldol reactions allowing the preparation of a key lactol intermediate to the atorvastatin side chain in a single step. Following substantial modification, this approach is now operated on an industrial scale to... 

The structural similarity between claenone (42) and stolonidiol (38) enabled Yamada to exploit an almost identical strategy for the total synthesis of (-)-stolonidiol (38) [40]. A short retrosynthetic analysis is depicted in Fig. 12. An intramolecular HWE reaction of 68 was successfully applied for the macrocyclization. The highly substituted cyclopentanone 69 was made available by a sequence that is highlighted by the sequential Michael-Mi-chael addition between the enolate 53 and the a, -unsaturated ester 70 followed by a retro-aldol addition. However, as is the case for the claenone (42) synthesis, the synthesis of stolonidiol (38) is characterized by numerous functional and protecting group transformations that are a consequence of Yamada s synthetic strategy. 

A recent approach toward ( )-guanacastepene (rac-187) published by Kwon in 2003 employed a sequential methyl cuprate 1,4-addition to the en-one 281 followed by an intramolecular Mukaiyama aldol condensation to construct the B ring (Eq. 9) [142, 143] ... 

J0rgensen has also reported a sequential Michael/Michael/aldol condensation for the three component coupling of malonitrile 111 and a,P-unsaturated aldehydes that involves two iminium ion catalysed Michael additions followed by an intramolecular aldol condensation (Scheme 43) [170]. Using diarylprolinol ether 55 (10 mol%) in a concentrated toluene solution of malonitrile 111 and 3 equivalents of a,P-unsaturated aldehyde the reaction products can be isolated in just 1 8 h (57-89% yield 97-99% ee). The atom efficiency of this three component reaction is remarkable and the ability to prepare these complex products under... 

This [1,4]-Wittig rearrangement system is applicable to the sequential [l,4]-rearrange-ment/aldol reaction which provides -hydroxy ketones with moderate diastereoselectivity (Table 4). 

According to equation 102, stereochemically homogeneous 3-carbonyl-substimted tetrahydrofurans are constructed in a brick-box system by sequential homoaldol and aldol reaction. The metallated aUyl carbamate serves as an equivalent for the chiral dianion A, which accepts two different aldehydes B and C in a highly controlled manner. ... 

When performing a Mannich reaction in its initial three-component design, the selectivity is sometimes difficult to obtain due to the competition with the side processes, primarily the auto-aldol condensation [52, 80], A common solution for this problem is the pre-formation of an imine or the enolate, or both and thus the sequential (indirect) performance of the reaction (Scheme 35) [52],... 

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