Jun-ichi Matsuo

The Mukaiyama Aldol Reaction: 40 Years of Continuous Development

A directed cross-aldol reaction of silyl enol ethers with carbonyl compounds, such as aldehydes and ketones, promoted by a Lewis acid, a reaction which is now widely known as the Mukaiyama aldol reaction. It was first reported in 1973, and this year marks the 40th anniversary. The directed cross-aldol reactions mediated by boron enolates and tin(II) enolates also emerged from the Mukaiyama laboratory. These directed cross-aldol reactions have become invaluable tools for the construction of stereochemically complex molecules from two carbonyl compounds. This Minireview provides a succinct historical overview of their discoveries and the early stages of their development.

Regioselective Inter- and Intramolecular Formal [4+2] Cycloaddition of Cyclobutanones with Indoles and Total Synthesis of (±)-Aspidospermidine†

Alkaloids and synthetic small molecules with a hydrocarbazole structure exhibit a broad range of important bioactivities. For the synthesis of structurally diverse and complex hydrocarbazoles, [4+2] cycloaddition to the C2–C3 positions of indoles has high synthetic potential, since two covalent bonds are formed in one step without the preintroduction of special substituents (e.g., a vinyl group ) on the indole. Such [4+2] cycloadditions of indoles are categorized into three types of Diels–Alder reaction (DAR): 1) normalelectron-demand DARs, in which strongly electron-deficient indoles that contain two electron-withdrawing groups at their 1and 3-positions are used, 2) inverse-electron-demand DARs of highly electron-poor dienes, such as 1,2,4,5tetrazine, and 3) DARs of photochemically induced radical cations with triarylpyrylium salts. In all of these DARs, the combination of indoles and enophiles (4p donors) is restricted, and it is also difficult to change the regioselectivity of the cycloaddition. Therefore, the development of a general, efficient, and conceptually new strategy for the synthesis of structurally diverse hydrocarbazoles still remains important and challenging. We describe herein the first example of a Lewis acid promoted formal [4+2] cycloaddition of cyclobutanones 6 with indoles (Scheme 2). This reaction enables the selective

Lewis Acid-Catalyzed Intermolecular [4 + 2] Cycloaddition of 3-Alkoxycyclobutanones to Aldehydes and Ketones

Intermolecular [4 + 2] cycloaddition between 3-alkoxycyclobutanones and aldehydes or ketones by the activation with boron trifluoride etherate is reported. The carbonyl compounds are inserted into the less substituted C2-C3 bond of the cyclobutanone ring of 6-alkyl-2-oxabicyclo[4.2.0]octan-7-ones to afford 1-alkyl-5,7-dioxabicyclo[4.4.0]decan-2-one derivatives regioselectively (>99:1) and diastereoselectively. On the other hand, [4 + 2] cycloaddition of 3-ethoxy-2,2-dialkylcyclobutanones at low temperature proceeds at the more substituted C2-C3 bond to yield 3,3-dialkyl-6-ethoxy-2,3,5,6-tetrahydro-4H-pyran-4-one derivatives with high regioselectivities. This [4 + 2] cycloaddition is developed into a one-pot synthesis of tri- or tetrasubstituted dihydro-gamma-pyrones from 3-ethoxycyclobutanones which are readily prepared from acid chloride and ethyl vinyl ether. The two regioselectivities observed in ring-opening of cyclobutanones can ascribe to thermodynamic stabilities of zwitterionic intermediates generated from tetrahydropyran-fused cyclobutanones and 3-ethoxycyclobutanones.

Stereocontrolled Formal Synthesis of (±)-Platensimycin

The caged structure of platensimycin, known as Nicolaous key intermediate for total synthesis of platensimycin, was synthesized stereoselectively by using the following key steps: (i) diastereoselective Diels-Alder reaction between gamma-benzoyloxy enone and tert-butyldimethylsiloxydiene, (ii) formation of a dihydropyran ring by intramolecular catalytic oxypalladation, and (iii) transannular radical cyclization of monothioacetal with tributyltin hydride and AIBN.

Tin(IV) Chloride Catalyzed Cycloaddition Reactions between 3-Ethoxycyclobutanones and Allylsilanes

Formal [4 + 2] cycloaddition between various 3-ethoxycyclobutanones and allyltrialkylsilanes proceeded to give 3-ethoxy-5-[(trialkylsilyl)methyl]cyclohexan-1-ones by catalysis with tin(VI) chloride. The use of allyl-tert-butyldiphenylsilane induced 1,5-hydride transfer, which gave 2-[3-(tert-butyldiphenylsilyl)propyl]-6-methyltetrahydro-4-pyrones.

A New Synthesis of 2,3-Di- or 2,3,3-Trisubstituted 2,3-Dihydro-4-pyridones by Reaction of 3-Ethoxycyclobutanones and N-p-Toluenesulfonyl Imines Using Titanium(IV) Chloride: Synthesis of (±)-Bremazocine

N-p-Toluenesulfonyl (Ts) aldimines reacted with 3-ethoxycyclobutanones by catalysis of titanium(IV) chloride to afford 2,3-di- or 2,3,3-trisubstituted N-Ts-2,3-dihydro-4-pyridones. Synthesis of (+/-)-bremazocine was efficiently accomplished by using this method.

Asymmetric Synthesis of 2,3-Dihydro-4-pyranones by Reaction of Chiral 3-Alkoxycyclobutanone and Aldehydes

Chiral cyclobutanone which had ethyl l-lactate as a chiral auxiliary at the 3-position reacted with aldehydes to give 2,3-dihydro-4-pyranones in up to 93% ee by combined use of titanium(IV) chloride and tin(II) chloride.

Brønsted acid catalyzed asymmetric reduction of ketones and acyl silanes using chiral anti-pentane-2,4-diol.

Ketones and acyl silanes were reduced to the corresponding alcohols by a simple procedure employing anti-1,3-diol and a catalytic amount (5 mol %) of 2,4-dinitrobenzenesulfonic acid in benzene at reflux. Asymmetric induction reached up to >99% ee when a chiral pentane-2,4-diol of 97% ee was used.

Oxazaborolidine-Catalyzed Enantioselective Reduction of α-Methylene Ketones to Allylic Alcohols

Oxazaborolidine-catalyzed enantioselective reduction of alpha-methylene ketones was efficiently carried out by using borane-diethylaniline as a stoichiometric reducing agent. The combination of this method and subsequent hydrogenation of thus-formed allylic alcohol improved stereoselectivity in the reduction of 24-oxocholesteryl ester to 24-(R)-hydroxycholesteryl ester.

Improved catalytic and stereoselective glycosylation with glycosyl N-trichloroacetylcarbamate: application to various 1-hydroxy sugars

Efficient catalytic and stereoselective glycosylation was achieved by activating a glycosyl N-trichloroacetylcarbamate with a catalytic amount of Lewis acid in the presence of a glycosyl acceptor and 5A molecular sieves. Catalytic one-pot dehydrative glycosylation of a 1-hydroxy carbohydrate was achieved stereoselectively by reaction with trichloroacetyl isocyanate, followed by activation with a catalytic amount of activators.