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Wacker-type reactions of terminal alkenes mainly lead to methyl ketones rather than aldehydes. The selective formation of aldehydes usually requires alkenes containing a suitable coordinating fragment. A few procedures however provide fair to high aldehyde/methyl ketone ratios in the absence of a directing group. This review, which is focused on the formation of aldehydes, summarizes the methods reported since 2007 and, with personal comments, the mechanisms and intermediates proposed to rationalize the regioselectivity.

A highly efficient method for the synthesis of 1,2,3-triaroylindolizines has been established from readily available methyl ketones and pyridines in the presence of iodine. This reaction also examplified the self-division of labor strategy in convergent domino synthesis.

The reactions of 5-pyrrolyl-pyrimidinyloxyacetaldehyde or methyl ketone with primary amines yielded hydroxymethylpyrrolopteridine derivatives via a cascade of iminium cyclization and O–N Smiles rearrangement. The present cascade exhibited a different profile compared to the previously reported ones, which consisted of N–N Smiles rearrangement. Lewis acid (TiCl4) under carefully controlled conditions was employed to suppress the competing formation of imine dimers to give the desired heterocycles. A plausible mechanism involving the iminium cyclization and Smiles rearrangement is proposed. This methodology has been used to generate a series of 6-hydroxymethylpyrrolo[1,2-f]pteridine derivatives with potential biological activities.

A facile two-step synthesis of natural germination inhibitor 5-butyl-3-oxo-2,3-dihydrofuran-2-yl-acetic acid [(+)-erigeronic acid A, 1] has been described via highly regioselective ring opening of (R)-acetoxysuccinic anhydride with the primary enolate of butyl methyl ketone, followed by an enzymatic hydrolysis and an in situ dehydrative cyclization pathway with 77% overall yield. On the basis of the present chemoenzymatic approach, (R)-configuration has been assigned to the C-2 chiral centre of the natural erigeronic acid.

Two coupling strategies for (−)-pre-swinholide A were devised based on the analysis in Scheme 1. In the first route, a boron-mediated aldol reaction between the ethyl ketone 19 and the aldehyde 3 was used to construct the C15-C16 bond with moderate diastereoselectivity. In the second route, a Mukaiyama aldol reaction between the methyl ketone 54 and the aldehyde 4 introduced the C18-C19 bond with complete stereocontrol.

A flexible and expedient synthesis of the leishmanicidal and trypanocidal monoterpene espintanol 1 , first isolated in 1991 from the spruce Oxandra espintana , was achieved in seven steps and with an overall yield of 11%. Key steps were the direct conversion of the methyl ester 6b to the methyl ketone 7 upon treatment with methyl lithium and the acid-catalyzed benzylic hydroperoxide rearrangement reaction of the secondary alcohol 11 to afford 1 .

A mixture of the methyl esters of communic acids ( 3b, 4b, 5b ) was used in the synthesis of the ambergris-type odorants Ambrox ® ( 1 ) and ambracetal ( 2 ). Both syntheses involve methyl ketone 7 as the key intermediate.

Stereocontrolled synthesis in racemic form of the title diterpene ethers is described. Friedel-Crafts acylation of the naphthalene derivative 15 afforded the methyl ketone 16 in high yield. The compounds 5 and 16 were converted into the hydrophenanthrenones 6 and 7 respectively. Reductive methylation of 6 and 7 in anhydrous ammonia furnished the β, γ -unsaturated ketones 8 and 9 which were stereoselectively transformed into the trans -fused ketones 10 and 11. Huang-Minlon reduction of 10 and 11 afforded the octahydrophenanthrene 27 and (±)-totaryl methyl ether (2) respectively. Friedel-Crafts acylation of 27 provided the methyl ketone 28 which was converted into (±)-semperviryl methyl ether (4).

Three epoxides (IX, XIX and XX) prepared from the eudesm-4-enes (VIII and XVIII) on treatment with BF 3 -etherate have been found to undergo skeletal rearrangement resulting in the formation of a number of ketones; i.e. 5- epi faurinan-4-one (X), 1( R )-isopropyl-3-(1′-methyl-5-oxohexylidene)cyclopentane (XI) and 1( S ),7( S )-dimethyl-4( R )-isopropyl-bicyclo[5.3.0]decan-6-one (XII) from 4α,5α-epoxy-10- epi eudesmane (IX), 1( R ),7( S )-dimethyl-4( R )-isopropyl-bicyclo[5.3.0]decan-6-one (XXI) from 4α,5α-epoxy-eudesmane (XIX), and the unsaturated methyl ketone (XI) from 4β,5β-epoxy-eudesmane (XX).

Dehydration of guaiol and the epimeric dihydroguaiols in the presence of many acidic dehydrating agents has been studied and the products characterized by GLC, spectroscopic analysis and by chemical transformations. The most interesting dehydration products are Bates' guaioxide (VIIIa, VIIIb) obtained by perchloric acid-acetic acid dehydration of guaiol (I); a methyl ketone (XIII) having a novel skeleton, obtained by the isomerization of the epoxide prepared from 1β,5β-dihydro-β-guaiene (XIb); and the secondary alcohol (XVI) formed by hydroboration-oxidation of 1α-,5α-dihydro-e-guaiene (XIIa).

Diketone 2,3,4,4a,5,6,7,7a,8,9,10,11-dodecahydro-2,10-dioxo-1 H -benzo[d]-naphthalene prepared by annulation of N-(1-cyclohexenyl)pyrrolidine with vinyl methyl ketone, was found to consist of two stereoisomers. Theoretical dipole moments of these were calculated, and their structures were shown by moment dipole measurements to be cis A/B, cis A/C and trans A/B, cis A/C, respectively. The isomer trans A/B, cis A/B was converted into trans A/B, cis A/C dodecahydro-1 H -benzo[d]naphthalene by the Wolff-Kishner reduction.