Practical Synthesis of Spirocyclic Bis-C,C-glycosides. Mechanistic Models in Explanation of Rearrangement Stereoselectivity and the Bifurcation of Reaction Pathways

BF3·OEt2-Catalyzed Unexpected Stereoselective Formation of 2,4-trans-Diallyl-2-methyl-6-aryltetrahydro-2H-pyrans with Quaternary Stereocenters

The present manuscript describes a convenient, mild, and highly stereoselective method for the allylation of δ-hydroxy-α,β-unsaturated ketones having a benzylic hydroxyl group at the δ-position using allyltrimethylsilane mediated by BF₃·OEt₂, leading to 2,4-diallyl-2-methyl-6-aryltetrahydro-2H-pyran ring systems with quaternary carbon stereogenic centers. This represents the first example of a tandem isomerization followed by one C-O and two C-C bond-forming reactions in one pot. The isolation of TMS-protected lactol as an intermediate from the reaction strongly supports the proposed mechanistic pathway.

Stereoselective synthesis of sugar fused β-disubstituted γ-butyro-lactones: C-spiro-glycosides from 1,2-cyclopropanecarboxylated sugars

A stereoselective methodology was developed for the construction of C-spiro-glycosides in two steps involving bromonium ion activated solvolytic ring opening of sugar derived 1,2-cyclopropanecarboxylates followed by a one-pot dehydrohalogenation, intramolecular hetero-Michael addition (IHMA) and ester hydrolysis. The obtained spirocyclic lactols were further converted to enantiomerically pure spirolactones.

Oxidative spirocyclisation routes towards the sawaranospirolides. Synthesis of ent-sawaranospirolides C and D

Two routes are described for the synthesis of the sawaranospirolides, stereoisomeric spirolactone ascorbigenins isolated from Chamaecyparis pisifera. Trapping of the keto enal formed by oxidation of a functionalised 2-(4-hydroxybutyl)furan affords a potential butenolide spiroacetal precursor to sawaranospirolides A and C. Alternatively, epoxidation of protected 3-(dihydropyran-2-yl)-3-arylpropanoic acids results in spirolactonisation to generate ent-sawaranospirolide C; a related acid-mediated spirocyclisation gave access to ent-sawaranospirolide D.

Formation and Reactivity of Novel Pyranosidic Nicholas Oxocarbenium Ions: Access toC-Ketosides and Branched-ChainC-Ketosides

[Structure: see text] Dicobalt hexacarbonyl propargyl complexes, prepared from alkynyl ketoses, display an unexpected reactivity when treated with Lewis acids in the presence of nucleophiles and furnish C-ketosides, branched-chain C-ketosides, or branched-chain C-glycals depending on the nucleophile and the carbohydrate starting material.

Asymmetric total synthesis of complex marine natural products

Among nature's ecosystems, the marine environment has been an extremely rich source of structurally complex and biologically active molecules. This review aims to cover the recent developments in the synthesis of marine natural products, also reflecting the trend of their increased use to address biological questions. The examples chosen should be viewed as representative of the different structural motifs on the one hand and the strategies and stimuli for their synthesis on the other.

Substitution and Remote Protecting Group Influence on the Oxidation/Addition of α-Substituted 1,2-Anhydroglycosides: A Novel Entry into C-Ketosides

[reaction: see text] C-Ketosides are valuable intermediates in chemical synthesis and as glycoside mimics. This manuscript describes the efficient generation of these substrates from alpha-alkyl-substituted glycals and an oxidative, C-C bond-forming sequence where the choice of C(3) protecting group was critical.

Investigations of alpha-siloxy-epoxide ring expansions forming 1-azaspirocyclic ketones

The construction of 1-azaspirocyclic cycloalkanones using a siloxy-epoxide semipinacol ring expansion process was examined. Functionalized 1-azaspiro[5.5]undecan-7-ones (1-azaspirocyclic cyclohexanones) proceeded in high chemical yields with complete diastereoselectivity using titanium tetrachloride as the Lewis acid promoter. The formation of functionalized 6-azaspiro[5.4]-decan-1-ones (1-azaspirocyclic cyclopentanones) proceeded in high chemical yield with little diastereoselectivity. Modification of reaction parameters such as the Lewis acid promoter or the nature of the silyl ether allowed for the preferential formation of either ("anti" or "syn" 1,2 alkyl shift) diastereomeric product. An explanation for the different reactivity profiles between the cyclobutanol silyl ethers and cyclopentanol silyl ethers is provided.

Synthesis of Functionalized 1-Azaspirocyclic Cyclopentanones Using Bronsted Acid or N-Bromosuccinimide Promoted Ring Expansions

Azaspirocyclic ring systems are present in a variety of alkaloids. Functionalized 1-azaspirocyclopentanones (6, 7, 11, 12) can be efficiently constructed through semipinacol ring expansion reactions of 2-(1-hydroxycyclobutyl)-p-toluenesulfonylenamides (4) promoted by either a Bronsted acid ((S)-(+)-10-camphorsulfonic acid or HCl) or N-bromosuccinimide, an electrophilic bromine source. Reactions promoted by N-bromosuccinimide tend to proceed in higher yields (80-95%) and with greater diastereoselectivity (3:1-1:0) compared to those reactions promoted by a Bronsted acid. In addition, N-bromosuccinimide promoted reactions can produce a complementary stereochemical outcome compared to the reactions using Bronsted acid.

Construction of azaspirocyclic ketones through alpha-hydroxyiminium ion or alpha-siloxy epoxide semipinacol rearrangements

[reaction: see text] Semipinacol-type rearrangements to produce azaspirocyclic ketones are presented. The yields and stereoselectivities of these reactions range from 67-94% yield and 2.8:1 to 1:0 diastereoselectivity, respectively.

1-Oxaspiro[4.4]nonan-6-ones. Synthetic access via oxonium ion technology, optical resolution, and conversion into enantiopure spirocyclic alpha,beta-butenolides

A general approach to the synthesis of enantiomerically pure spirocyclic alpha,beta-butenolides is presented where the fundamental framework is rapidly elaborated by acid- or bromonium ion-induced rearrangement of the carbinol derived by addition of 2-lithio-4,5-dihydrofuran to cyclobutanone. Subsequent resolution of the resulting ketones by either sulfoximine or mandelate acetal technology has been applied effectively. The availability of these building blocks makes possible in turn the acquisition of the enantiomers of dihydrofurans typified by 17, 35, and 38 and lactones such as 25 and 31, as well as the targeted title compounds. Complementary reductions of the early intermediates provide the added advantage that the alpha- and beta-stereoisomeric carbinol series can be obtained on demand. These capabilities have been coordinated to allow the crafting of any member of the series in relatively few steps.

An Enantioselective Ring Expansion Route Leading to Furanose and Pyranose Nucleosides Featuring Spirodiketopiperazines at the Anomeric Position

A study directed at the enantioselective synthesis of spirodiketopiperazine homologues of hydantocidin is described. Furanoid glycals, systems that are amenable to C-5 metalation in the presence of tert-butyllithium, are readily coupled to N-protected 2,3-azetidinediones provided that at least 1 equiv of BF(3).OEt(2) is present to curb enolization. The resulting 1:1 mixtures of carbinols undergo smooth ring expansion to spirocyclic keto amides when heated with pyridinium p-toluenesulfonate in benzene. 1,2-Acyl shifts operate exclusively. Since attempts to engage these products in Beckmann rearrangement proved singularly unsuccessful, recourse was alternatively made to new methodology based upon sequential Baeyer-Villiger oxidation and ammonolysis. The data show that the first of these steps occurs with exclusive migration of the quaternary carbon. Furthermore, nucleophilic attack by NH(3) can be directed regioselectively to the anomeric region. If heating is supplied during acid-promoted cyclization to the spirodiketopiperazines, spiropyranose derivatives are produced in a complementary process. The central issue of this synthesis effort was the utilization of 4-phenylseleno-substituted furanoid glycals so as to ultimately enable introduction of the cis-diol functionality at C-3 and C-4 (hydantocidin numbering).