Spiro-chirality center

Another route to a methyl-branched derivative makes use of reductive cleavage of spiro epoxides ( ). The realization of this process was tested in the monosaccharide series. Hittig olefination of was used to form the exocyclic methylene compound 48. This sugar contains an inherent allyl alcohol fragmenC the chiral C-4 alcohol function of which should be idealy suited to determine the chirality of the epoxide to be formed by the Sharpless method. With tert-butvl hydroperoxide, titanium tetraisopropoxide and (-)-tartrate (for a "like mode" process) no reaction occured. After a number of attempts, the Sharpless method was abandoned and extended back to the well-established m-chloroperoxybenzoic acid epoxida-tion. The (3 )-epoxide was obtained stereospecifically in excellent yield (83%rT and this could be readily reduced to give the D-ribo compound 50. The exclusive formation of 49 is unexpected and may be associated with a strong ster chemical induction by the chiral centers at C-1, C-4, and C-5. 

Other possible cases of chiral tetracoordinate centers exist beside the asymmetric centers (point group C,) discussed above. Indeed, chiral molecules of higher symmetry exist these may contain a chiral center of the types Z(a2b2) (point group C2), Z(a3b) (point group C3), or Z(a4) (point group D2). An example of C2 chirality is provided by (S)-( — )-spiro[4.4]nonane-l,6-dione (XVI) [33]. An in-depth discussion... 

Diastereoselective allylzincations of achiral cyclopropenone acetals, addressing the control of the relative configuration at the newly formed C-C bond (centers C3 and C4) and of chiral derivatives such as 20 (R = H), addressing chirality transmission through a spiro carbon center, have been investigated. In the presence of bis(oxazoline) ligands, enantioselective reactions of this kind can also be performed. ... 

The efEcient, highly enanhoselective construction of quaternary carbon centers on P-keto esters under phase-transfer conditions has been achieved using N-spiro chiral quaternary ammonium bromide 9h as a catalyst [46]. This system has a broad generality in terms of the structure of P-keto esters and alkyl halides (Scheme 11.11). The resultant alkylahon products 54 can be readily converted into the corresponding P-hydroxy esters 55 and P-amino esters 56, respechvely. 

Segment C (21) is a relatively small molecule and is synthesized from 8. Acyclic stereocontrol for the construction of the C-29 chiral center was very important. Compound 8 was first converted to the lactone (22), which was treated with the carbanion of 23 to give 24. Removal of the sulfone with aluminum amalgam, spiro-ketalization, and Peterson olefination gave 25, which was treated with methyllithium at - 78 °C in tetrahydrofuran. Syn-addition controlled by an a-chelation occurred smoothly to give the expected segment C (21) in high yield [8a, b]. 

In order to obtain asymmetric spiro compounds, there are two different possibilities. First, one can connect two different chromophores via a common spiro center. The thiophene compounds 39a and 39b are one example [84, 85]. Second, one can connect two equal but asymmetric chromophores. Based on this principle are Spiro-PBD (40), spiro-bridged bis(phenanthrolines) (41) [86], and the branched compounds Octol (42a) and Octo2 (42b) [87]. Because of their symmetry, these molecules are chiral. The glass transition temperatures of 40 and 42b are reported to be 163 and 236°C, respectively [88], Unfortunately, reports on the thermal properties of 39 and 41 are lacking. 

Currently, the chiral phase-transfer catalyst category remains dominated by cinchona alkaloid-derived quaternary ammonium salts that provide impressive enantioselec-tivity for a range of asymmetric reactions (see Chapter 1 to 4). In addition, Maruoka s binaphthyl-derived spiro ammonium salt provides the best results for a variety of asymmetric reactions (see Chapters 5 and 6). Recently, some other quaternary ammonium salts, including Shibasaki s two-center catalyst, have demonstrated promising results in asymmetric syntheses (see Chapter 6), while chiral crown ethers and other organocatalysts, including TADDOL or NOBIN, have also found important places within the chiral phase-transfer catalyst list (see Chapter 8). 

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

The reaction of 13, in which the (3-pyridyl)carbonyloxy groups can be either in syn or in anti position, with spiro[cyclopropene-3,9 -fluorene] creates two new stereogenic centers.35 Thus two diastereomers are possible for each the syn- and the anri-isomer which form a pair of C2-symmetrical enantiomers (R,R/S,S) and a Cs- or Cj-symmetrical meso form (R,S) n The resulting calixarenes 14, bearing dihydroindolizine units, were studied as chromogenic compounds ( calixo-chromes ) in quenching experiments not related to their chirality. 

---