R. J. F. Nivard

Photodehydrocyclizations in stilbene-like compounds—III : Effect of steric factors

Abstract Photodehydrocyclizations in stilbene-like compounds take place only if the sum of the free valence numbers of the atoms concerned in the cyclization step exceeds in the excited state, a value of 1. If more than one cyclization is possible the photoconversion follows the pathway for which ΣF* is maximal; if ΔΣF* for various cyclizations is smaller than 0·1 a second product is sometimes formed. These rules also bold if only non planar and no planar products can arise. If planar as well as non planar compounds can be formed the main product is as a rule a planar polycyclic aromatic, even if ΣF* for its formation is lower than for other cyclizations. Sometimes but not always, a non planar product (with higher ΣF*) is found as a side product. The applicability of these rules in the planning of synthetic procedures for polycyclic aromatics is illustrated in the preparation of several new compounds (e.g. the four isomeric benzo hexahelicenes).Abstract Photodehydrocyclizations in stilbene-like compounds take place only if the sum of the free valence numbers of the atoms concerned in the cyclization step exceeds in the excited state, a value of 1. If more than one cyclization is possible the photoconversion follows the pathway for which ΣF* is maximal; if ΔΣF* for various cyclizations is smaller than 0·1 a second product is sometimes formed. These rules also bold if only non planar and no planar products can arise. If planar as well as non planar compounds can be formed the main product is as a rule a planar polycyclic aromatic, even if ΣF* for its formation is lower than for other cyclizations. Sometimes but not always, a non planar product (with higher ΣF*) is found as a side product. The applicability of these rules in the planning of synthetic procedures for polycyclic aromatics is illustrated in the preparation of several new compounds (e.g. the four isomeric benzo hexahelicenes).

Photodehydrocyclizations in stilbene-like compounds—II : Photochemistry of distyrylbenzenes

Abstract The photochemistry of distyrylbenzenes has been reinvestigated. It appears that formation of 1,12-benzperylene from the para isomer and picene from the ortho compound occur via an indirect route; the relevant cyclizations take place only in dimers of the parent compounds. Other photoproducts from ortho distyrylbenzene are stilbene and phenanthrene, probably arising via 2,3-diphenyl-2,3-dihydronaphthalene. These results show that the photoreactivity of the three isomers is entirely in accordance with the general rules previously formulated for photodehydrocyclizations in stilbene-like compounds.

Conformational studies on helicenes—V : Alteration of the conformation of helicenes by annelation of benzo groups☆

Abstract Analysis of the NMR spectra of five heptahelicenes and some related compounds reveals that annelation of heptahelicene by one or more benzo groups results in a conformational change of the helix. The extent and direction of this change was deduced correctly from the NMR spectra by application only of the ring current theory and the influence of Van der Waals forces, as could be established from the X-ray analyses of heptahelicene and the tribenzo derivative 12.

Comparison of the applicability of 2-hydroxyethylsulfonylmethyl- and chloromethyl-polystyrenes in the solid-phase synthesis of protected peptides

Abstract In order to compare the applicability of resins in the preparation of protected peptides two solid-phase syntheses of a protected ACTH-(5–10)-hexapeptide have been performed, one using 2-hydroxyethylsulfonylmethyl-polystyrene and the other with Merrifields chloromethylated polystyrene. To obtain a good comparison, equivalent methods were used as far as possible. Optimal conditions for deprotection of amino groups and for the liberation of the end-product were determined. Chromatographic examination of the crude fission products of the peptidyl resins presented important clues towards the nature of the underlying fission mechanism. Using an automated peptide synthesizer the pure end-product was obtained from both resins in approximately the same yield (ca. 70%). It appeared that the isolation of the product from the suffone resin is less laborious, since the critical alkali-treatment, necessary for the liberation of the product, proceeds faster and, if properly carried out, avoids transesterification.

Photodehydrocyclizations of stilbene-like compounds—XI : Synthesis and racemization of the double helicene diphenanthro[4.3-a; 3′.4′-o]picene

Abstract The synthesis of a new double helicene, diphenanthro[4.3-a;3′.4′-o]picene, is described. Of the two possible diastereoisomers only one was obtained as a racemate. The compound was partly resolved with TAPA yielding fractions enriched with one or the other enantiomer. They could not be converted into the other diastereomer by heating, but racemized under these conditions at a relatively low temperature and with a rate comparable to that of the racemization of hexahelicene at the same temperature. The NMR spectrum is discussed.

The influence of the Π-electron distribution and Π -bond stability of ketene acetals on their reactivity and stereoselectivity in thermal (2 + 2) cycloadditions with 1,1-dicyanostyrenes

Abstract The reactions of a series of ketene acetals (R1R2C C(OMe)2) with dicyanostyrenes (ArCH C(CN)2) have been studied. Ketene acetals with a symmetrical Π-electrone distribution give stable cyclobutanes via reactions in which no dipolar intermediates can be trapped. Ketene acetals with an unsymmetrical Π-electron distribution give unstable cyclobutaes or other products derived from dipolar intermediates. In this case, dipolar intermediatess can be intrecepted with H2O. The results have been explained by supposing a different approach of the reactant in the cycloadditions of the two different types of ketene acetals Further support for this supposition was obtained from the stereochemical course of the cycloadditton rcactions with R1HC C(OMe)2. (R1 OMe, Me, Cl) and from the Hammett plots which show a better correlation with σ+for the symmetrically substituted (MeO)2C(OMe)2 and with σ for the unsymmetrical MeCH C(OMe)2.

Conversion of N-hydroxytryptophans into α,β-dehydrotryptophan. An approach to the neoechinulin series

Reaction of the N-hydroxytryptophan derivative (21b) with pyruvoyl chloride gives access to the N-hydroxydioxopiperazine (28b). O-Tosylation of the latter compound, followed by base treatment, affords the dioxopiperazine (30b), an N(1)-allyl derivative of the fungal metabolite neoechinulin B (6). The biogenetic relevance of this reaction sequence is discussed.

Chemistry of ketene acetals. Part 9. A simple ‘one-pot’ synthesis of 4-hydroxy-δ-lactones and 5,6-dihydro-2-pyrones from 1,1-dimethoxypropene and β-oxy aldehydes

Protected β-oxy aldehydes react easily with 1,1-dimethoxypropene (1a) in the presence of ZnCl2 to give 2,2-dimethoxyoxetanes. Hydrolysis of these oxetanes and deprotection of the latent hydroxy function in a one-pot procedure gives 4-hydroxy-δ-lactones in moderate to good yields. Starting with β-oxy aldehydes having an α-branched side chain and defined stereochemistry, δ-lactones with completely defined stereochemistry can be synthesized. Dehydration of the hydroxy lactones with concentrated sulphuric acid gives easy access to 5,6-dihydro-2-pyrones. The usefulness of this route is demonstrated in the synthesis of a simple, optically active 5,6-dihydro-2-pyrone (37).

Chemistry of tetra-alkoxyethenes. Part VI. Cycloadditions with αβ-unsaturated carbonyl compounds and chemistry of the resulting tetra-alkoxydihydropyrans

Heating tetra-alkoxyethenes (1) at 100°C with α-cyano-αβ-unsaturated carbonyl compounds (2) gives [4 + 2] cycloaddition products in high yields. Mild acidic hydrolysis of the resulting 2,2,3,3-tetra-alkoxy-5-cyano-3,4-dihydro-2H-pyrans (4) gives separable mixtures containing a δ-oxo-ester (11), formed by ring opening, and an α-pyrone (12), which arises by recyclization of the δ-oxo-ester after hydrolysis of its acetal function. In concentrated sulphuric acid, compounds (4) are converted either into 3,3-dialkoxy-5-cyano-3,4-dihydro-2-pyrones (13) or into 3-alkoxy-5-cyano-2-pyrones (14), depending on the reaction temperature. Treatment of (4) with base yields 2,2,3-trialkoxy-5-cyano-2H-pyrans (15) by elimination of alcohol.

Chemistry of ketene acetals. Part 8. Stereochemistry of the reaction of 1,1-dimethoxypropene with aldehydes

The Lewis acid-catalysed (2 + 2)cycloadditions of 1,1-dimethoxypropene with various aldehydes RCHO have been studied. These reactions, which are supposed to proceed via dipolar intermediates, yield 2,2-dimethoxyoxetanes, the formation of which is reversible in the presence of the catalyst used. The cis:trans ratio of the oxetanes was determined after short reaction times at low temperature (‘kinetic’ conditions) and after prolonged reaction times at room temperature (thermodynamic conditions). This was performed by stopping the reaction with triethylamine or by transforming the oxetanes at low temperature into β-hydroxy esters. The influence of the reaction conditions and the size of R on the stereochemistry of the reaction has been discussed. It appeared that under ‘kinetic’ conditions the cis:trans ratio of the oxetanes is not only determined by the most favourable transoid approach of the cycloaddends, but also by the rotation of the dipolar intermediate to a cisoid gauche conformation.