Spiro skeleton

Another approach to shift absorption bands for the colored form is the extension of Jt-conjugation outside the spiro skeleton. Procedures of molecular designs for such photochromic compounds are shown in Scheme 20. (1) Position for extension of additional -conjugation in spirothiopyran... 

The flexible, semi-flexible, or anancomeric behavior of tetrahydro-l,3-oxazines 83 spiro-connected to cyclohexane was studied by NMR spectroscopy. The possible stereoisomers were deduced by taking into account the special chirality of the heterocycle and the spiro skeleton <2000M975>. 

This again illustrates the problem of correct citation of hydrogenation. The unsubstituted spiro skeleton is structure a, named as spiro[oxiran-2,3 (2 //)-pyridine], it being necessary to introduce an indicated hydrogen (2 H) to allow for the spiro linkage. The ketone groups are... 

Hydantocidin, which bears a spiro skeleton, is a natural product and a herbicide. Eq. 3.113 shows the typical preparation method of spiro-diketone (277) from the reaction of a-diazoketone (276) with Rh2(OAc)4 via a carbenoide species. However, this spiro skeleton can be also constructed by radical reactions. Treatment of (3-keto ester (278) with Mn(OAc)3 in the presence of electron-rich olefin generates spiro-cyclic thioketal (279) through the formation of a (3-keto ester radical, intermolecular radical addition to... 

A bis-methylene compound of a spiro skeleton structure 21 is polymerized to produce a poly(ether carbonate) 22 (Eq. (16)) I3 I4). The course of the polymerization has been explained by a scheme involving free-radical intermediates 23 and 24. 

Isoxazoline rings include two heteroatoms which could serve as Lewis bases. Chiral spiro bis(isoxazoline) ligands, named SPRIXs, bearing a chiral spiro skeleton and two isoxazoline rings, have been reported <19990L1795>. 

The above syntheses of the parent spiroamine system establish the skeleton of the aromatic alkaloids. A synthesis of a derivative of jS-erythroidine (III) with the intact spiro skeleton, achieved in Boekel-heide s laboratory (15), provides similar confirmation for the lactonic alkaloids. 

The most attractive detailed hypotheses (Fig. 11) suggest the formation of the erythrinane skeleton by oxidation of LXXXVIII, a symmetrical intermediate derived from two molecules of tyrosine or dihydroxyphenylalanine. The two additional bonds necessary might be formed in either order. In one hypothesis (1, 57) oxidation of one aromatic ring to the o-quinone (LXXXIX) is followed by nucleophilic addition of the amino group and further oxidation to XC (or the related o-quinone). This sequence is exactly analogous to the in vitro oxidation of dihydroxyphenylalanine itself to the quinone dopachrome (3S). Nucleophilic or radical addition of the second phenolic ring to the quinoid system would complete the spiro skeleton of XCI. 

The total syntheses of (+)-secosyrins 1 and 2 was achieved and their relative and absolute stereochemistry was unambiguously established by C. Mukai and co-workers. To construct the spiro skeleton of these natural products, the intramolecular Nicholas reaction was utilized. The alkyne substrate was first converted to the dicobalt hexacarbonyl complex by treatment with Co2(CO)8 in ether. Exposure of the resulting complex to boron trifluoride etherate at room temperature brought about the ring closure with inversion of configuration at C5 to afford the expected tetrahydrofuran derivative. The minor product was the C5 epimer which was formed only in 15% yield. 

Asymmetric domino cyclization of the unsaturated alcohol 71 proceeded smoothly to give the bicyclic compound 72 with 82% ee in 89% yield as a single diastereomer using a spiro bis(oxazoline) ligand which has a chiral spiro skeleton and two oxazoline rings. The reaction proceeds via the formation of 73 and 74 as oxypalladation products. BQ was used as a reoxidant [36],... 

For an excellent review on the use of spirocyclic ligands see Ding, K., Han, Z., Wang, Z. (2009). Spiro skeletons a class of privileged structure for chiral hgand design. Chemistry An Asian Journal, 4, 32-41. 

The synthesis of novel CILs with a spiro skeleton has been reported by Sasai and coworkers [97]. The influence of Al-substituents and the counteranions on their chiral discrimination abilities was investigated. In addition, the diastereomeric interaction between the novel spiro imidazolium-based CIL 17 and (5)-Mosher s potassium salt was examined. The H NMR spectrum of racemic spiro imidazolium salt 17 exhibited two pairs of counteranion from bromide to a chiral anion. The counteranion was changed in situ by treating racemic 17 with the potassium salt of Mosher s acid in the presence of 18-crown-6. The NMR spectrum exhibited excellent splitting in each pair of doublets. These results demonstrate spiro imidazolium salts are potentially useful in chiral molecular recognition (Fig. 13). 

The side chain in spirostan derivatives (44) can undergo certain exchange reactions which are characteristic for this skeleton. In general, this spiro-ketal moiety is quite stable under alkaline or neutral conditions, but in acidic media it undergoes a reversible opening of ring F (45 and 46). In... 

At first, acid-catalyzed cyclization of Hofmann degradation products was undertaken however, the cyclization proceeded via the 5-exo-trigonal mode instead of the 6-endo-trigonal mode, resulting in no benzo[c]phenanthridine skeleton. Dyke and Brown (114-117) reinvestigated Perkin s results (118,119) and established the structure of the cyclized products 196 and 197 derived from the methine base 194 (Scheme 36). Onda et al. (120,121) obtained the five-membered spiro compounds 198 and 199 by treatment of 195 with dilute hydrochloric acid. 

Tephrosone 33, a flavonoid belonging to the tephrorin family (vide supra), was found to possess a dihydrobenzofuran framework . Moreover, a metabolite of Penicillium brevicompactum, namely brevione A 34 was found to contain a spiro system . Neomarinone 35, another metabolite from a marine actinomycete, constitutes an intriguing sesquiterpenoid skeleton <00TL2073>. 

A new selective thermal cascade ring-enlargement process of 4-chloro-substituted spiro[cyclopropane-l,5 -isoxazolidines], leading to a new method for the synthesis of the indolizine skeleton, has been reported (see Scheme 17). Apparently, the process is made possible by the presence of a chlorine substituent on the carbon a to the spirocyclopropane ring which facilitates a cyclopropyl-to-cyclobutyl ring enlargement mediated by a polar solvent. 

The spiro(indanedione) anion-radical in Scheme 3.67 was studied by ESR and UV/visible spectroscopy (Maslak et al. 1990). The spectra clearly indicated delocalization of the spin density over the entire molecnlar framework. An unpaired electron undergoes simultaneous delocalization between the halves (in the ESR timescale). The observed spectra were independent of the counterions (Li, Na, and K+), thus excludiug auy iou-pairiug complicatiou. As a geueral iufereuce, an unpaired electron spends its time on both the half-shaped orbitals with no geometrical changes in the molecular skeleton of this anion-radical. 

Illudins M 112 and S 113 are sesquiterpenes produced by Omphalotus illu-dens, the jack-o -lantern mushroom [149]. These compounds demonstrated in vitro selective toxicity for a wide range of tumor cells compared to normal cells, but poor therapeutic indices were found when tested in vivo [150]. The spiro-cyclopropane and a, -unsaturated ketone moities present in the illudin skeleton constitute a bis-electrophile that is responsible for the DNA damage (vide supra Sect. 2.6) [150,151]. Illudin derivatives with greatly improved therapeutic indices have been prepared first of all dehydroilludin M 114 has shown better... 

Zezschwitz and co-workers controlled the stereochemistry of hydroxyl spiro [4.5] acetal skeleton of 13 and 14 utiHzing intramolecular hydrogen bonding between the secondary hydroxyl group and pyran oxygen (Scheme 7.5) [9]. 

The cyclic ammonium ylide/[l,2]-shift approach has been successfully applied by West and Naidu to a key step in the total synthesis of (—)-epilupinine, one of the biologically active lupin alkaloids. Cu(acac)2-catalyzed diazo decomposition of enantiomeric pure diazoketone 160 in refluxing toluene generates a spiro ammonium ylide 161 and 162, which then undergoes [l,2]-shift to give rise to a quinolizidine skeleton as a mixture of diastereomers (95 5) (Scheme Major diastereomer 164 has enantiomeric purity of 75% ee. The partial retention of stereo-... 

The spiro (4,4) nonanediols (1,6) represent an appropriate example for the study of the relations of internal hydrogen bond strength and 0 0 distances (resp. 0 H distances) [2]. One would expect a rather rigid carbon skeleton of spirononane, so that the 0 0 distance may be estimated from molecular models. 

These compounds range through monocyclic (with a bisabolane framework), bicyclic (with an eudesmane, amorphane, axane, guaiane, isodaucane, or gorgonane framework), tricyclic (with an aromadendrane, cubenane, maaliane, pupukeanane, or trachyopsane framework), and spiro[4,5]decane carbon skeletons. 

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