Hydroxyacids as efficient chiral spacers for asymmetric intramolecular [2+2] photocycloadditions

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent aspects of [2 + 2] photocycloaddition chemistry with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in sections 2 and 4 , whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin.

Transparent Films from CO2 -Based Polyunsaturated Poly(ether carbonate)s: A Novel Synthesis Strategy and Fast Curing

Transparent films were prepared by cross‐linking polyunsaturated poly(ether carbonate)s obtained by the multicomponent polymerization of CO₂, propylene oxide, maleic anhydride, and allyl glycidyl ether. Poly(ether carbonate)s with ABXBA multiblock structures were obtained by sequential addition of mixtures of propylene oxide/maleic anhydride and propylene oxide/allyl glycidyl ether during the polymerization. The simultaneous addition of both monomer mixtures provided poly(ether carbonate)s with AXA triblock structures. Both types of polyunsaturated poly(ether carbonate)s are characterized by diverse functional groups, that is, terminal hydroxy groups, maleate moieties along the polymer backbone, and pendant allyl groups that allow for versatile polymer chemistry. The combination of double bonds substituted with electron‐acceptor and electron‐donor groups enables particularly facile UV‐ or redox‐initiated free‐radical curing. The resulting materials are transparent and highly interesting for coating applications.

A catalytic tethering strategy: simple aldehydes catalyze intermolecular alkene hydroaminations

Herein we describe a catalytic tethering strategy in which simple aldehyde precatalysts enable, through temporary intramolecularity, room-temperature intermolecular hydroamination reactivity and the synthesis of vicinal diamines. The catalyst allows the formation of a mixed aminal from an allylic amine and a hydroxylamine, resulting in a facile intramolecular hydroamination event. The promising enantioselectivities obtained with a chiral aldehyde also highlight the potential of this catalytic tethering approach in asymmetric catalysis and demonstrate that efficient enantioinduction relying only on temporary intramolecularity is possible.

[2 + 2]-Photocycloaddition of 1,1-Diethoxyethylene to Chiral Polyfunctional 2-Cyclohexenones. Regioselectivity and π-Facial Discrimination

The photochemical [2 + 2]-cycloadditions of 1,1-diethoxyethylene to chiral polyfunctional 2-cyclohexenones have been carried out leading to the production of highly constrained unusual alpha-amino acids with excellent regioselectivity and satisfactory yields. Theoretical calculations have been done to rationalize the observed regio- and diastereoselectivity and show that regiochemistry is determined by the relative rate of formation of the 1,4-biradical intermediates and not by the stability of these species.

Synthesis of enantiopure 3-azabicyclo[3.2.0]heptanes by diastereoselective intramolecular [2+2] photocycloaddition reactions on chiral perhydro-1,3-benzoxazines

[2+2] photocycloadditions involving chiral 3-acryloyl-2-vinylperhydro-1,3-benzoxazines derived from (-)-8-aminomenthol are highly diastereoselective reactions. The facial selectivity depends on the type of substitution at the vinyl double bond, and always leads to cis-fused bicyclic derivatives. The de is good for compounds with one substituent at the outer carbon of the double bond at C-2, but only one diastereomer is formed in cyclizations of compounds with two substituents at that position. The elimination of the menthol appendage gives enantiopure 3-azabicyclo[3.2.0]heptanes.

Asymmetric intramolecular [2 + 2] photocycloadditions: alpha- and beta-hydroxy acids as chiral tether groups

Chiral alpha- and beta-hydroxy acids such as (S)-lactic acid, (S)-phenyllactic acid, (S)-mandelic acid, or (3R)-3-hydroxybutyric acid have been used as tether groups for intramolecular and diastereoselective [2 + 2] photocycloaddition of 3-oxocyclohexene carboxylic acid derivatives. Total regiocontrol toward the straight adduct and high diastereoselectivities (up to 94%) were observed in the case of butenyl lactate 11. After separation of the two diastereoisomers, cleavage of the chiral tether under basic conditions afforded cyclobutane lactones in good yield and enantiomeric pure form. An X-ray structure has been recorded that confirmed the relative and absolute configuration of the three contiguous stereogenic centers assigned according to CD spectra.