Mild Lewis Acid Catalyst Research Articles

A surprising dipolar cycloaddition provides ready access to aminoglycosides.

This contribution describes the results of a new research effort in our laboratory aimed at the synthesis of novel aminoglycosides and amino-C-glycosides. Despite the importance of such compounds, and the previous development of some methodological solutions, this remains an important area of research. Notable features of our approach, which is distinct from and complementary to previous efforts, are the following: (1) Reliance on a surprising and unprecedented formation of glycal triazolines via an inverse electron demand dipolar cycloaddition of glucal. We believe this desirable transformation has not previously been discovered because of the unusual selection of substrates and solvent required. (2) Very mild reaction conditions. An initial thermal cycloaddition is carried out in an inert solvent, the triazoline generated is photochemically converted to a reactive aziridine, and the crude aziridine undergoes ring opening at room temperature in the presence of a nucleophile and a mild Lewis acid catalyst. (3) Formation of products lacking an N-acyl group, allowing ready synthesis of novel glucosamine derivatives.

Formation of Porphyrins in the Presence of Acid-Labile Metalloporphyrins: A New Route to Mixed-Metal Multiporphyrin Arrays

The ability to incorporate distinct metalloporphyrins at designated sites in multiporphyrin arrays is essential for diverse applications in materials and biomimetic chemistry. The synthesis of such mixed-metal arrays via acid catalyzed reactions has largely been restricted to metalloporphyrins of stability class II (e.g., Cu, Co, Ni) or I. We describe routes for the rational synthesis of mixed-metal arrays via acid-catalyzed condensations that are compatible with metalloporphyrins of stability class III (e.g., Zn) and IV (e.g., Mg). The routes are demonstrated for p-phenylene-linked arrays. The key finding is that several mild Lewis acids [InCl(3), Sc(OTf)(3), Yb(OTf)(3), and Dy(OTf)(3)], which are known to catalyze the dipyrromethane + dipyrromethane-dicarbinol condensation in CH(2)Cl(2) at room temperature without acidolysis, do not demetalate zinc or magnesium porphyrins under the same conditions. Rational routes to porphyrin dyads and triads employ reaction of a (porphyrin)-dipyrromethane and a (porphyrin)-dipyrromethane-dicarbinol. The porphyrin-forming reactions (six examples) proceed in yields of 18-28%. The metalation states of the arrays prepared in this manner include Zn-free base (ZnFb), MgFb, ZnFbMg, ZnFbZn, and ZnFbFb. Studies of the catalysis process indicate that the dipyrromethane + dipyrromethane-dicarbinol condensation is catalyzed by both the Lewis acid and a Brønsted acid derived in situ from the Lewis acid. Taken together, the ability to employ otherwise "acid-labile" metalloporphyrins as precursors in condensation procedures should broaden the scope of accessible mixed-metal multiporphyrin arrays and motivate further studies of the application of mild Lewis acid catalysts in porphyrin chemistry.

Indium Triflate: A Mild Lewis Acid Catalyst for Thioacetalization and Transthioacetalization.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Indium triflate: a mild Lewis acid catalyst for thioacetalization and transthioacetalization

Protection of a variety of carbonyl compounds as thioacetals using indium triflate, a mild Lewis acid catalyst, was achieved at ambient temperature in very good yield. Transthioacetalization of oxyacetals into thioacetals was also achieved in an excellent yield. A mixture of carbonyl compound and its respective oxyacetal was also completely converted into thioacetal in the presence of indium triflate.

High pressure catalyzed addition of lactams to siloxydienes

Lactams add to Danishefsky's diene 1 yielding N-alkenyl substituted lactams 3-5. The reaction occurs only under pressure (100-300MPa) and, preferentially, in the presence of a mild Lewis acid catalyst such as bismuth chloride or ytterbium triflate. The reactions are fully regioselective and afford linear trans multifunctional 1:1 adducts.

Iron(III) 2-ethylhexanoate as a novel, stereoselective hetero-Diels–Alder catalyst

Iron(III) 2-ethylhexanoate has been used as a novel, mild Lewis acid catalyst for the stereoselective Diels–Alder reaction of ethyl (E)-4-oxobutenoate with alkyl vinyl ethers to stereoselectively produce cis-2-alkoxy-3,4-dihydro-2H-pyran-4-carboxylic acid, ethyl esters with diastereoisomeric excesses (de) as high as 98%.

A Facile One Stage Synthesis of Oxazolines Under Microwave Irradiation

Abstract A rapid, high yielding procedure for the synthesis of 2-substituted oxazolines has been achieved under microwave irradiation from alkyl and aryl nitriles and β-amino alcohols using a mild Lewis acid catalyst.

Tellurium tetrachloride as a mild and efficient catalyst for dithioacetalization

A variety of aldehydes and aliphatic ketones were converted into the corresponding dithioacetals at room temperature in good yields by using small amounts of tellurium tetrachloride as a mild Lewis acid catalyst.

Regio- and stereo-selectivity in uncatalysed and catalysed Diels–Alder reactions of allenic esters with furan and 2-methylfuran

Complete regio- and stereo-selectivity in both uncatalysed and catalysedt Diels–Alder additions of furan to the allenic esters (1a–e), and of 2-methylfuran to the allenic esters (1b, c) has been determined through detailed 1H and 13C NMR spectral investigations. Eu(fod)3 and Pr(fod)3 have been found to be very useful mild Lewis acid catalysts for these cycloadditions; several mol % of these reagents significantly enhance the selectivity and yield of these reactions without adversely affecting the fragile functionalities. A rationalization of the observed regio- and stereoselectivity and reactivity profile, based on the stereoelectronic factors operating in the transition states is given.

Mechanisms of Formation of Cyclic Urea Nucleosides. 2. Direct N-Glycosylation Versus O- to N-Transglycosylation

Abstract Cyclic, saturated urea nucleosides, can be formed either by a direct N-glycosylation mechanism or by an intermolecular 0- to N-transglycosylation process in the presence of a mild Lewis acid catalyst such as (CH3)3SiSO2CF3(TMSOTf). The mechanistic preference is controlled by the type of interaction between the persilylated ureas (1a,b) and TMSOTf which appears to be solvent dependent. Previously made tetramethyleneurea 0-nucleoside (7b) rearranged very efficiently (86%) to the corresponding N-nucleoside 6b in the presence of TMSOTf.

General route to methoxy-substituted arylphosphonous dichlorides via mild Lewis acid catalysts

General route to methoxy-substituted arylphosphonous dichlorides via mild Lewis acid catalysts