Traceless sulfone linker cleavage triggered by ozonolysis: solid-phase synthesis of diverse α-β-unsaturated carbonyl compounds

Traceless Solid-Phase Organic Synthesis

Traceless solid-phase synthesis represents an ultimate sophisticated synthetic strategy on insoluble supports. Compounds synthesized on solid supports can be released without a trace of the linker that was used to tether the intermediates during the synthesis. Thus, the target products are composed only of the components (atoms, functional groups) inherent to the target core structure. A wide variety of synthetic strategies have been developed to prepare products in a traceless manner, and this review is dedicated to all aspects of traceless solid-phase organic synthesis. Importantly, the synthesis does not need to be carried out on a linker designed for traceless synthesis; most of the synthetic approaches described herein were developed using standard, commercially available linkers (originally devised for solid-phase peptide synthesis). The type of structure prepared in a traceless fashion is not restricted. The individual synthetic approaches are divided into eight sections, each devoted to a different methodology for traceless synthesis. Each section consists of a brief outline of the synthetic strategy followed by a description of individual reported syntheses.

Efficient 2-sulfolmethyl quinoline formation from 2-methylquinolines and sodium sulfinates under transition-metal free conditions

An efficient approach for 2-sulfolmethyl quinoline formation from 2-methylquinolines and sodium sulfinates is described.

Solid-phase synthesis of diverse spiroisoxazolinodiketopiperazines

A convenient, efficient protocol to prepare diverse spiroisoxazolino-diketopiperazines via a parallel solid-supported synthesis was developed. The key steps are (1) a coupling reaction of an amino acid; (2) tosylation with concomitant β-elimination to form an α, β-unsaturated ester; (3) a 1,3-dipolar cycloaddition with an oxime to form isoxazoline rings; and (4) cyclic cleavage to release the product from the resin. All reaction steps and workup procedures were modified to allow the use of automated or semiautomated equipment. A 100-member demonstration library with two diversity sites was prepared in good purity and acceptable overall yields.

Nucleophilicity and nucleofugality of phenylsulfinate (PhSO(2)(-)): a key to understanding its ambident reactivity

Second-order rate constants for the reactions of the phenylsulfinate ion PhSO(2)(-) with benzhydrylium ions Ar(2)CH(+) have been determined in DMSO, acetonitrile, and aqueous acetonitrile solution using laser-flash and stopped-flow techniques. The rate constants follow the correlation equation log k (20 degrees C) = s(N + E), which allows the determination of the nucleophile-specific parameters N and s for PhSO(2)(-) in different solvents. With N = 19.60, PhSO(2)(-) is a slightly weaker nucleophile than malonate and azide ions in DMSO. While PhSO(2)(-) reacts with highly stabilized benzhydrylium ions to give benzhydryl phenyl sulfones exclusively, highly reactive benzhydrylium ions give mixtures of sulfones Ar(2)CH-SO(2)Ph and sulfinates Ar(2)CH-OS(O)Ph; the latter rearrange to the thermodynamically more stable sulfones through an ionization recombination sequence. Sulfones generated from PhSO(2)(-) and stabilized amino-substituted benzhydrylium ions undergo heterolysis in aqueous acetonitrile and the rate of formation of the colored benzhydrylium ions was followed spectrophotometrically by stopped-flow techniques. The ranking of the electrofugalities of the benzhydrylium ions (i.e., the relative ionization rates of Ar(2)CH-SO(2)Ph) was not the inverse of the ranking of their electrophilicities (i.e., the relative reactivities of Ar(2)CH(+) with nucleophiles), which was explained by differences in Marcus intrinsic barriers. While sulfones are thermodynamically more stable than the isomeric sulfinates, the intrinsic barriers for the attack of benzhydrylium ions at the oxygen of PhSO(2)(-) are significantly lower than the intrinsic barriers for S-attack, and the activation energies for the attack of carbocations at sulfur are only slightly smaller than those for attack at oxygen. Because reactions of PhSO(2)(-) with carbocations of an electrophilicity E > -2 (i.e., carbocations which are more reactive than Ph(3)C(+)) are diffusion-controlled, the regioselectivities of the reactions of PhSO(2)(-) with "ordinary" carbocations do not reflect relative activation energies.