Thioacetic acid, Michael addition

A recent total synthesis of tubulysin U and V makes use of a one-pot, three-component reaction to form 2-acyloxymethylthiazoles <06AG(E)7235>. Treatment of isonitrile 25, Boc-protected Z-homovaline aldehyde 26, and thioacetic acid with boron trifluoride etherate gives a 3 1 mixture of two diastereomers 30. The reaction pathway involves transacylation of the initial adduct 27 to give thioamide 28. This amide is in equilibrium with its mercaptoimine tautomer 29, which undergoes intramolecular Michael addition followed by elimination of dimethylamine to afford thiazole 30. The major diastereomer serves as an intermediate in the synthesis of tubulysin U and V. 

The same group utilized thiourea 12 (10mol% loading) for the catalysis of the enantioselective Michael addition of thioacetic acid to various chalcones [214]. At room temperature and otherwise unchanged conditions, in comparison to the... 

Scheme 6.64 Michael adducts provided from the 12-catalyzed asymmetric addition of thioacetic acid to various chalcones.

Scheme 6.65 Mechanistic proposals for the biflinctional mode of action of catalyst 12 in the Michael addition of thioacetic acid to nitroalkenes (A) and to chalcones (B).

Cinchona alkaloids and their derivatives have been reported to catalyse the Michael addition of (V-heterocycles, such as benztriazole, to nitroalkenes in moderate to high enantioselectivities (<94% ee) 15 The thiourea derivative (149) catalysed Michael addition of thioacetic acid to a range of frafts-/f-nitrostyrenes to afford RCH(SAc)- CH2NO2 (<70% ee) 16 The thiourea derivative (149) and its congeners have been identified as efficient organocatalysts for the Michael addition of a-substituted cyano-acetates RCH(CN)C02Et to vinyl sulfones CH2=C(R)S02Ph (72-96% ee) 17 ... 

Lanthanide triflates and Sc(OTf)3 effectively catalyze conjugate addition of SEE, KSA, and ketene silyl thioacetals under mild conditions (0°C to room temperature, 1-10 mol% catalyst) (Scheme 10.86) [69, 238]. After an aqueous work-up these Lewis acids can be recovered almost quantitatively from the aqueous layer and can be re-used without reduction of fheir catalytic activity. Eu(fod)3 also is effective in not only aldol reactions but also Michael addition of KSA [239]. The Eu(fod)3-catalyzed addition of KSA is highly chemoselective for enones in the presence of ketones. 

Some thermally forbidden [2 + 2]-cycloaddition reactions can be promoted by Lewis acids1-6. With chirally modified Lewis acids, the opportunity for application in asymmetric synthesis of chiral cyclobutanes arises (for a detailed description of these methods see Sections D.l. 6.1.3.. D.l. 61.4. and references 7, 28-30). Thus, a chiral titanium reagent, generated in situ from dichloro(diisopropoxy)titanium and a chiral diol 3, derived from tartaric acid, catalyzes the [2 + 2]-cycloaddition reaction of 2-oxazolidinone derivatives of a,/ -unsalurated acids 1 and the ketene thioacetal 2 in the presence of molecular sieves 4 A with up to 96 % yield and 98% ee. Fumaric acid substrates give higher yields and enantiomeric excesses than acrylic acid derivatives8. Michael additions are almost completely suppressed under these reaction... 

Michael addition of thioacetic acid to a series of a-substituted (V-acryloyloxazolidin-2-ones, followed by enantioselective protonation, catalysed by the cinchonidine-derived thiourea (288a), has been reported to proceed with <97% ee The pseudo-enantiomeric, cinchonine-derived thiourea (289a) can catalyse the Michael addition of dimedone to enone RCH=CHCO( -Py) with <98% ee The isosteviol-derived thiourea (290) represents yet another variation this organocatalyst has been reported to facilitate the Michael addition of a-substituted cyanoacetates NCCH(Ar)C02R and maleimides in toluene at -30 °C (with <93% ee and <98 2 dr) as a method for the construction of quaternary chiral centres " ... 

Hydroxy-a-phenylthio-acids and their methyl esters can be prepared by condensations between carbonyls and the bis-anionic species (13) from phenyl-thioacetic acid or the monoanion (14) from methyl phenylthioacetate respectively. Yields are often excellent although the bis-anion (13) is less reactive, failing to condense with 2-methylcyclohexanone or acetophenone. With enones, (13) undergoes [1,2] addition by contrast the monanion (14) adds in a Michael fashion. 

Asymmetric chiral (thio)urea-catalyzed sulfa-Michael reactions have been extended to thioacetic acid and nitroal-kenes (Scheme 46.33). In 2006, Wang and co-workers used Takemoto s chiral thiourea 272 in the addition reaction of thioacetic acid 271 to acyclic enones 270 with modest enan-tioselectivities ranging from 20% to 1Q%P The use of... 

Li H, Zu L, Wang J, Wang W. Organocatalytic enantioselec-tive Michael addition of thioacetic acid to enones. Tetrahe-dronLett. 2006 47 3145-3148. 

The squaramide-based thiourea (422) has been reported to catalyse the Michael addition of thioacetic acid to nitroalkenes (423) (R =Ar, hetero-Ar, alkyl R =Me, Et,... 

The Michael type of addition, in the presence of acids, has been extended successfully to cyclopropyl ketones347 (reaction 86). Of course, the functional group can also undergo subsequently a transformation202,346,347,349, compatible with the phosphonium structure (acetalization, thioacetalization, imination, etc.). 

The utihty of Cu(II)-box complex 96 for asymmetric Mukaiyama-Michael reaction has been intensively studied by Evans et al. (Scheme 10.91) ]248]. In the presence of HFIP fhe 96-catalyzed reaction of S-t-butyl thioacetate TMS enolate with alkylidene malonates provides fhe Michael adducts in high chemical and optical yield. HFIP plays a crucial role in inducing catalyst turnover. Slow addition of the silyl enolate to a solution of 96, alkylidene malonates, and HFIP is important in achieving high yields, because fhe enolate is susceptible to protonolysis with HFIP in fhe presence of 96. The glutarate ester products are readily decarboxylated to provide chiral 1,5-dicarbonyl synthons. Quite recenfly, Sibi et al. reported enantioselective synthesis of t -amino acid derivatives by Cu( 11)-box-catalyzed conjugate addition of silyl enolates to aminomefhylenemalonates ]249]. 

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