Terpenoids—LXIX

Enantioselective Ir-Catalyzed Hydrogenation of Terminal Homoallyl Sulfones: Total Synthesis of (-)-Curcumene

A novel methodology for the preparation of chiral methyl benzylic compounds is reported. Terminal homoallyl sulfones were prepared from homoallyl alcohols, which are easily accessible through the recently reported Lewis acid isomerization of oxetanes. The iridium-catalyzed asymmetric hydrogenation of homoallylic sulfones afforded γ-chiral sulfones with excellent enantioselectivities (up to 98% ee). The synthetic potential of this novel methodology was demonstrated by the total synthesis of (R)-(-)-curcumene.

Application of The Dess-Martin Oxidation in Total Synthesis of Natural Products

Dess-Martin periodinane (DMP), a commercially available chemical, is frequently utilized as a mild oxidative agent for the selective oxidation of primary and secondary alcohols to their corresponding aldehydes and ketones, respectively. DMP shows several merits over other common oxidative agents such as chromiumand DMSO-based oxidants; thus, it is habitually employed in the total synthesis of natural products. In this review, we try to underscore the applications of DMP as an effective oxidant in an appropriate step (steps) in the multi-step total synthesis of natural products.

Tandem Peterson olefination and chemoselective asymmetric hydrogenation of β-hydroxy silanes

Tandem Peterson olefination and asymmetric hydrogenation of β-hydroxy silanes provides an efficient route to access the chiral benzylic hydrocarbon. The chemoselectivity can be tuned by the addition of base.

Here, we report the first Ir–N,P complex catalyzed tandem Peterson olefination and asymmetric hydrogenation of β-hydroxy silanes. This reaction resulted in the formation of chiral alkanes in high isolated yields (up to 99%) and excellent enantioselectivity (up to 99% ee) under mild conditions. Modification of the reaction conditions provides a choice to transform either an olefin or the β-hydroxy silane in a chemoselective manner. Additionally, based on this method, an expedient enantioselective synthesis of (S)-(+)-α-curcumene, from a simple ketone, was accomplished in two steps with 75% overall yield and 95% ee.

Total synthesis of natural productsviairidium catalysis

An overview of the highlights in total synthesis of natural products using iridium as a catalyst is given.

Asymmetric decarboxylative 1,4-addition of malonic acid half thioesters to vinyl sulfones: highly enantioselective synthesis of 3-monofluoromethyl-3-arylpropanoic esters

An asymmetric decarboxylative 1,4-addition of malonic acid half thioesters (MAHTs) to 2-aryl-substituted vinyl sulfones has been developed, yielding adducts with excellent enantioselectivity (up to 97 % ee). In view of tuning pKa values, a quinine-based benzyl-substituted thiourea was designed and demonstrated as the most efficient catalyst. The enantioselective synthesis of 3-monofluorinated analogues of 3-methyl indanone and (+)-turmerone has been accomplished from decarboxylative 1,4-addition adducts with satisfactory results.

Acetylcholinesterase inhibitory activity of volatile oil from Peltophorum dasyrachis Kurz ex Bakar (yellow batai) and Bisabolane-type sesquiterpenoids

In this study, the chemical compositions and acetylcholinesterase (AChE) inhibitory activitiy of the volatile oil from the bark of Peltophorum dasyrachis Kurz ex Bakar (yellow batai) were evaluated. As a result, 68 compounds, accounting for 88.0% of the total oil, were identified. The main characteristic constituent in P. dasyrachis was isolated by silica gel column chromatography and found to be a sesquiterpenoid, (+)-(S)-ar-turmerone (1). In the AChE inhibitory assay, the volatile oil showed potent inhibitory activity with the IC(50) value of 83.2 +/- 2.8 microg/mL. Among the volatile oil components and characteristic sesquiterpenoids, (+)-(S)-ar-turmerone (1) and (+)-(S)-dihydro-ar-turmerone (2) were potent compounds, inhibiting AChE in a dose-dependent manner, with IC(50) values of 191.1 +/- 0.3 and 81.5 +/- 0.2 microM, respectively. (+)-(S)-Dihydro-ar-turmerone (2), in particular, was found to be the most potent AChE inhibitor. Also, bisabolane-type sesquiterpenoid derivatives, (+)-(7S,9S)-ar-turmerol (3), (+)-(7S,9R)-ar-turmerol (4), (+)-(7S,9S)-dihydro-ar-turmerol (5), (+)-(7S,9R)-dihydro-ar-turmerol (6), (+)-(S)-ar-curcumene (7), and (+)-(S)-dihydro-ar-curcumene (8), were synthesized and tested for their AChE inhibitory effect, and their structure-activity relationships were evaluated. All sesquiterpenoids exhibited AChE inhibitory activity. The order of AChE inhibitory potency by bisabolane-type sesquiterpenoids was as follows: ketones > alcohols > hydrocarbons. Furthermore, the inhibition kinetics analyzed by Dixon plots indicated that (+)-(S)-ar-turmerone (1) is a competitive inhibitor, with a K(i) value of 882.1 +/- 2.1 microM, whereas (+)-(S)-dihydro-ar-turmerone (2) is a non-competitive inhibitor.

Synthesis of aromatic bisabolene natural products via palladium-catalyzed cross-couplings of organozinc reagents

Aromatic bisabolene derivatives were prepared by two methods involving cross-coupling of organozinc reagents. The first synthesis of (+/-)-glandulone A (10), as well as syntheses of (+/-)-curcuhydroquinone (8) and (+/-)-curcuquinone (9), were accomplished via coupling of a secondary alkyl zinc reagent (1,5-dimethyl-4-hexenylzinc halide, 18) to protected bromohydroquinones using Pd(dppf)Cl(2) as catalyst. Coupling of arylzinc halides with alkenyl triflate 16 using Pd(PPh(3))(4) catalyst provided a number of bisabolene derivatives and led to syntheses of dehydro-alpha-curcumene (2), (+/-)-curcuphenol (3), and (+/-)-elvirol (13). A high-yield synthesis of the (+/-)-heliannuol D precursor 29 is also reported using this method.