Five contiguous

The structure of carpanone (1) was revealed by Australian scientists in 1969.1 Carpanone, a hexacyclic molecule and host of five contiguous stereogenic centers, is a lignan found in the bark of the car-pano tree. 

In the event, treatment of a rapidly stirred solution of 3 and sodium acetate in MeOH-tbO at 38 °C with PdCl2 results in the fomation of carpanone (1) in 46% yield. The ordered unimolecular transition state for the oxidative coupling reaction furnishes putative bis(quinodimethide) 2 stereoselectively. Once formed, 2 readily participates in an intramolecular Diels-Alder reaction4 to give carpanone (1). Two new rings and all five contiguous stereocenters are created in this spectacular sequential transformation.5... 

The construction of the five contiguous stereocenters required for a synthesis of compound 3 is now complete you will note that all of the substituents in compound 5 are positioned correctly with respect to the carbon backbone. From intermediate 5, the completion of the synthesis of the left-wing sector 3 requires only a few functional group manipulations. Selective protection of the primary hydroxyl group in 5 as the corresponding methoxymethyl (MOM) ether, followed by benzylation of the remaining secondary hydroxyl, provides intermediate 30 in 68 % overall yield. It was anticipated all along that the furan nucleus could serve as a stable substi-... 

The enol ether double bond contained within the ds-fused dioxa-bicyclo[3.2.0]heptene photoadducts can also be oxidized, in a completely diastereoselective fashion, with mCPBA. Treatment of intermediate XXII, derived in one step from a Patemo-Buchi reaction between 3,4-dimethylfuran and benzaldehyde, with mCPBA results in the formation of intermediate XXIII. Once again, consecutive photocycloaddition and oxidation reactions furnish a highly oxygenated system that possesses five contiguous stereocenters, one of which is quaternary. Intermediate XXIII is particularly interesting because its constitution and its relative stereochemical relationships bear close homology to a portion of a natural product known as asteltoxin. 

Extension of this tandem process to create five contiguous stereogenic centers has been accomplished by using 2-substituted vinyl ethers (Eq. 8.114).178 The results for the cycloaddition... 

The key problem in the preparation of 296, 297 undoubtedly is the stereoselective assembly of the chains of four to five contiguous cyclopropyl groups. The strategies of all total syntheses known so far rely on auxiliary-directed stereoselective cyclopropanations of double bonds [111a, 163,164]. The chemistry of bicyclopropylidene (1), namely its capability to be functionalized (Schemes 8, 9), dimerized (Scheme 15), and stereoselectively reduced (Scheme 53) opens up new perspectives to approach the assembly of quatercyclopropyl derivatives. 

A different [3+2] cycloaddition occurs on reaction of certain vinyldiazoacetates with styryl ethers [30], Although the mechanistic details are not known, the reaction is considered to be a concerted process and furthermore, it displays very high levels of stereocontrol. As illustrated in the Rh2(S-DOSP)4-catalyzed reaction shown in Eq. (22), the [3+2] cycloadduct 42 is formed as the all-czs di-astereomer in 98% ee. Cuprate addition to 42 stereoselectively generates the cyclopentane 43 with five contiguous stereocenters. 

A direct aldol-Tishchenko reaction of aromatic aldehydes with ketones proceeds with stereocontrol of up to five contiguous centres in a chain, using titanium(IV) r-butoxide and cinchona alkaloids.146 A tricyclic transition state is proposed to explain the high (g) degree of stereoselection. 

The calculated isothermal elastic tensor for yS-HMX is compared in Table 8 to the one reported by Zaug (isentropic conditions). Uncertainties in the calculated elastic coefficients represent one standard deviation in values predicted from five contiguous two nanosecond simulation sequences from the overall ten nanosecond simulation. As mentioned above, Zaug s experiments sufficed to determine uniquely five of the thirteen elastic constants (modulo the... 

In the solid state, X-ray crystallography (Blank et al., 1971) and Raman spectroscopy (Isbrandt and Oertei, 1980) data suggest that the dodecasodium salt of phytic acid adopts the sterically hindered 5 ax/1 eq conformation. Again, the 5 ax/1 eq form may be adopted to minimize electrostatic repulsion between the five contiguous dianionic phosphates in equatorial positions. 

Control of acyclic stereochemistry. The five contiguous stereocenters of aldehyde 5, an intermediate in Kishi s total synthesis of monensin (6), were established by the hydroboration reactions of 1 and 3, which afforded 2 and 4, respectively, with high stereoselectivity (8 1 diastereomer ratio for 1 12 1 for 3). Kishi argues that the... 

The first total synthesis of barbacenic acid, a bisnorditerpene containing five contiguous stereocenters, was achieved by A. Kanazawa et. al. They started out from a Wieland-Miescher ketone analogue that could be synthesized with high yield and excellent enantioselectivity by the procedure of S. Takahashi. According to this procedure, the Michael addition product 2-methyl-2-(3-oxo-pentyl)-cyclohexane-1,3-dione was cyclized in the presence of (S)-(-)-phenylalanine and D-camphorsulfonic acid. 

LiOAc ) and/or reaction conditions are involved. 1,3-Dinitropropane and especially 2-substituted homologues, react with enals to provide 2,4-dinitrDcyclohexanols with up to five contiguous stereocenters and ee up to 94%. ... 

This reaction on extension to 4 provides 5 with five contiguous chiral centers in 95% yield. 

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