The Unusual Stereochemistry of the Cycloaddition of Vinyl Ethers with Sulfonyl Isothiocyanates

Abstract

fluoroborate in high yields to the crystalline, stable, highmelting imidazolium salts (3a-d). On treatment with dilute sodium hydroxide the salts (3a-d), readily soluble in dimethylformamide, exchange their 2-halogen substituents for hydroxy groups selectively and almost quantitatively. With NaOH the intermediary 2-hydroxyimidazolium salts in turn split off NaBF, to afford the 4,5-dihalo-2(3 H)-imidazolones (4a-d) (cf. UV irradiation (benzene, filter il=350 nm) of (4a, b) without carefully excluding oxygen (or far more rapidly on passing through 0,) leads in high yields to the dialkylparabank acids (5a, b)[9al, most probably via the intermediary endoperoxides"'], which spontaneously eliminate chlorine. If oxygen is completely excluded, e.g. in argon atmosphere, (4b) is photoarylated (benzene, filter , I = 320 nm); (6) and (7) can be detected (each 3% yield) with the aid of mass spectrometry. ~CH;-CH;The sole isolable Diels-Alder adduct (8) of (4d) is formed with ethyl sorbate on heating for 8 h in benzene under inert gas. Careful work-up affords colorless crystals which rapidly decompose. (8) is characterized spectroscopically (see Table 1); the mass spectrum shows a molecular ion (triplet) of weak intensity at m/e=436. Procedure (2a-d):A suspension of anhydrous potassium carbonate in a solution of ( l a , c) (100 mmol) in anhydrous acetone (300 cm3) is treated with 100 mmol dialkyl sulfate and the mixture heated for 2 h under reflux. After filtration, evaporation of the filtrate to dryness, and treatment of the residue with ethanol,

Consecutive and Selective Double Methylene Insertion of Lithium Carbenoids to Isothiocyanates: A Direct Assembly of Four‐Membered Sulfur‐Containing Cycles

“…Aside of an unsubstituted phenyl ring (11), the incorporation of halogen such as fluorine (12,17,18), bromine (13), chlorine (14-17) at different positions of the ring are compatible with the methodology and, their presence could be advantageously exploited for further elaboration (vide infra). sp 3 -Hybridized hydrocarbons do not alter the efficiency of the technique (19)(20)(21), including when an ipropyl group (22) or the bicyclic indanyl system (23) were installed on the heterocumulene starting material. Preparing imino-thietanes with the proposed strategy proceeded with remarkable chemocontrol, as documented by isothiocyanates decorated with unsaturated residues: no concomitant carbenoid-mediated cyclopropanation [32] took place on a vinylsubstituted analogue (24), neither deprotonation of a terminal alkyne (25) despite the basic conditions.…”

“…To the best of our knowledge, such a structural characteristic is unprecedented among the scattered imino-thietanes previously described (Scheme 1path e). Indeed, uniformly-the recyclization of b-lactones (Mulzer), [19] the cycloaddition of ketenimines to thioketones (Dondoni), [20] the cycloaddition of vinyl ethers with sulfonyl isothiocyanates (Schaumann), [21] and, the photochemical isomerization of N-monosubstituted a,b-unsaturated thioamides (Sakamoto) [22] -afforded imino-thietanes variously substituted on the CÀC linkage. [23] 4-Ethoxyisothiocyanate 1 was selected as the model substrate for the reaction optimization (Table 1).…”

Consecutive and Selective Double Methylene Insertion of Lithium Carbenoids to Isothiocyanates: A Direct Assembly of Four‐Membered Sulfur‐Containing Cycles

“…To the best of our knowledge, such a structural characteristic is unprecedented among the scattered imino‐thietanes previously described (Scheme 1 — path e ). Indeed, uniformly—the recyclization of β‐lactones (Mulzer), [19] the cycloaddition of ketenimines to thioketones (Dondoni), [20] the cycloaddition of vinyl ethers with sulfonyl isothiocyanates (Schaumann), [21] and, the photochemical isomerization of N ‐monosubstituted α,β‐unsaturated thioamides (Sakamoto) [22] —afforded imino‐thietanes variously substituted on the C−C linkage. [23]…”