We have accomplished the first total synthesis of an isomerically pure naphthoquinonopyrano-γ-lactone dimer, γ-actinorhodin, in eleven steps. Two steps exploit pairs of peri-MeO groups as unusual selectivity controls. The respective MeO groups convey the steric bulk of a bromo or iodo substituent located ortho to one MeO group as steric hindrance into the vicinity of the second MeO group. This relay effect was indispensable for exerting regiocontrol in an aromatic bromination and diastereocontrol in an oxa-Pictet-Spengler cyclization. The absolute configuration of our target compound was established in an asymmetric Sharpless dihydroxylation of a β,γ-unsaturated ester, which was synthesized in a Heck coupling of a bromoiodonaphthalene with ethyl vinylacetate. The dihydroxylation provided the γ-hydroxylactone moiety of the bromonaphthalene that was used as the substrate in the oxa-Pictet-Spengler cyclization. Dimerization to the core of γ-actinorhodin occurred by two Suzuki couplings.

Since Sharpless' discovery of the asymmetric dihydroxylation of C=C double bonds in the late 1980s this reaction has become a powerful tool of synthetic organic chemistry. As a consequence, this transformation has been reviewed repeatedly and extensively. The present microreview focuses on Sharpless' asymmetric dihydroxylations (from here on “SADs”) of β,γ‐unsaturated carboxylic esters. These SADs differ from most others in that they provide not nonracemic 1,2‐diols but follow‐up products thereof. Bearing ester groups at appropriate distances, the SAD‐based 1,2‐diols obtained from β,γ‐unsaturated carboxylic esters lactonize readily under SAD conditions. Accordingly, SADs of β,γ‐unsaturated carboxylic esters furnish nonracemic β‐hydroxy‐γ‐lactones in a single operation. We show that this SAD route represents a – or even the most – potent easy‐to‐handle route to nonracemic butanolides and butenolides “of almost all kinds”. The span covered is from pioneering racemic work to applications in natural product synthesis. Some non‐butanolide and non‐butenolide target syntheses are included, especially those of tetrahydrofurans.

3,4‐Dimethoxybenz‐1‐yne and 2‐siloxylated furans without or with a bromine atom at C‐3 undergo Diels–Alder reactions with orientational selectivity. Hydrolysis furnished a bromine‐free or a bromine‐containing naphthalene, respectively. Bromination of the former provided a regioisomer of the latter. Either of the two compounds was processed to give a variety of unnatural naphthoquinonopyrano‐γ‐lactones. This occurred by a succession of (1) Heck coupling, (2) asymmetric dihydroxylation, (3) oxa‐Pictet–Spengler cyclization, and (4) oxidation. The fifteen monomeric naphthoquinonopyrano‐γ‐lactone structures that we prepared resemble the natural product (–)‐arizonin C1 or its C‐5 epimer. Accordingly, they represent hexasubstituted naphthalenes likewise. The sixteenth naphthoquinonopyrano‐γ‐lactone that we synthesized is a kind of dimer. Its moieties are bridged differently than those in naturally occurring naphthoquinonopyrano‐γ‐lactone dimers.

Uns gelang die erste Totalsynthese von einem isomerenreinen dimeren g-Lacton-anellierten Pyranonaphthochinon, (+ +)-g-Actinorhodin. Unsere elf Stufen umfassende Synthese nutzt zwei Mal ein Paar peri-ständiger Methoxygruppen als Mediatoren einer ungewçhnlichen Selektivitätssteuerung durche inen weit entfernten Brom-oder Iodsubstituenten. Dessen Sperrigkeit gaben die Methoxygruppen an die beiden Naphthalinpositionen weiter,a nd enen wir Ar-S E -Reaktionen vornahmen. Das ermçglichte bei einer Kernbromierung eine perfekte Regiokontrolle und bei einer Oxa-Pictet-Spengler-Cyclisierung die gewünschte Diastereokontrolle.Die Absolutkonfiguration unseres Zielmoleküls legte die Sharpless-Dihydroxylierung eines b,g-ungesättigten Esters fest, der einer Heck-Kupplung entstammte.S ie ergab den g-Hydroxylactonteil eines Bromnaphthalins,d er als Substrat der Oxa-Pictet-Spengler-Reaktion fungierte.Z um g-Actinorhodingerüst führte eine dimerisierende Suzuki-Kupplung.Abbildung 1. Natürliche und synthetische g-Lacton-anellierte Pyranonaphthochinone.[*] Dipl.-Chem.M .Neumeyer,Prof. Dr.R.B r ückner

Previous total syntheses of arizonin C1 (4) led to opposite assignments of its absolute configuration. Here, we report the fourth total synthesis thereof. In addition, we disclose the first total synthesis of arizonin B1 (3) proceeding differently than via arizonin C1. The stereocenters of the two targets stemmed from an asymmetric dihydroxylation and an ensuing oxa‐Pictet–Spengler cyclization. Their configurations were in line with Fernandes' assignments. Protective‐group variation in the substrate modulated the diastereoselectivity of the Pictet–Spengler cyclization between 77:23 in favor of a trans disubstitution at C‐3a vs. C‐5 – used for preparing the natural (–)‐arizonins C1 and B1 – and 100:0 in favor of a cis disubstitution – exploited for synthesizing the unnatural (+)‐5‐epi‐arizonins C1 and B1. All naphthalenes of the present study were derived from the (benzyloxy)methoxynaphthalenediol 19. It resulted from a Diels–Alder reaction of the aryne 17a with the siloxyfuran 18a.

Review: [146 refs.

No abstract

No abstract