Acyclic Derivative

The results of 14 structural determinations of acyclic pentacoordinate phosphorus derivatives using diffraction and microwave techniques are listed in Table 2.3.1. Electron diffraction studies on members of the series PFs Me , n = 0-3, the results of which are illustrated in Figs. 2.1.1. and 2.3.1., were extensively commented upon in Sections 2.1. and 2.2., on account of their relevance to the development of bonding theory for pentacoordinate phosphorus. The following general stereochemical features of acyclic derivatives may be recognised on the basis of the presently available information  

All the molecules are trigonal bipyramids. No considerable distortion towards a C4v structure for PX4Y or a Cs structure for PX3Y2 derivatives (X more electronegative than Y) is observed. 

Compound / (P-C) Axial Equatorial X r(P-X) Axial Equatorial Method of investigation Ref. 

4) The equatorial distances show a statistically significant difference. 

The ligand distribution over the axial and equatorial sites of the trigonal bipyramid for all unambiguous structural determinations is in accordance with electronegativity predictions. An apicophilicity series which differs in order from that based on ligand electronegativities has not as yet been established on the basis of diffraction studies. 

Carbohydrates are synthesized in plants from CO2 and H2O via a multistep biochemical process, the stoichiometry of which can be expressed by the following general equation  

The alkyne 49 reacted with samarium diiodide in the presence of Pd(0) (conditions that usually produce allenes) producing ene-yne 50 as the major product (47%) with only 9% of the expected allene.  

As noted in Section 2.5 above, a general approach to enol ethers from nitro-alkenes, as depicted by the conversion of 37 into 38, has also been applied to the preparation of a range of acyclic derivatives. Here, the pyranose rings (of 37 and 38) were replaced with various benzyl or cyclohexylidene protected L-glycero- 

In a useful variant of the Wittig olefination process, unprotected aldoses reacted in dioxane at around reflux temperature with stabilized ylides of the type Ph3P=CHC02R, where R is a bulky substituent (Bu , CHPhz), giving high yields of acyclic products with -selectivity. No cyclic products resulting from intramolecular Michael reaction were formed.  

Structural features of pyranoses unprotected at 0-1 are important in determining product outcome on reaction with the anion derived from diethylmethyl- 

The synthesis of the 2-phenylthio-hept-2-enoic acid 53, a precursor of 2-keto-3-deoxy-D-arai //io-heptonic acid has been achieved by addition of the anion derived from PhSCH2C02Me to 2,3 4,5-di-0-cyclohexylidene-D-arabinose.  

Hayashi, H. Kawabata, S. Shimono and A. Kakehi, Tetrahedron Lett., 2000,41, 2591. 

Kleban, U. Kautz, J. GreuI, P. Hilgers, R. Kugler, H.-Q. Dong and V. Jager, Synthesis, 2000,1027. 

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Cyclic diesters are often even better substrates forlipases and esterases than acyclic derivatives. Small-ring monoacetates (28, n — 1-3) are obtained in higher yield and ee than the larger derivatives (for 28, n = 4 is only 50%) (43). Hydrolysis of tetrahydrofuran diester results in monoester (29) of ee > 99% (44). 

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

It is also evident from the data of Bly et al. (95), Jacobs and Macomber (91), and Garry and Vessiere (99) that neopentyl-type homoallenic systems do not yield cyclopropyl derivatives upon solvolysis, in contrast to the unsubstituted parent system. If they have no substituents at Cj or C3, neopentyl homoallenic substrates yield rearranged acyclic olefins and rearranged solvent-incorporated products exclusively. If they carry an alkyl substituent at Ci, they give both rearranged and unrearranged acyclic products. If a substituent is present on C3 besides the acyclic derivatives, cyclobutyl products also are formed. 

There is not one generic route into 1,3,2-dithiazolyl/ium chemistry. For acyclic derivatives the [4+2] cycloaddition chemistry of [SNS][AsF6] to alkynes has been successfully expoited by Passmore.49 Flowever for benzo-fused derivatives, the simplest route is via Wolmershauser s method in which 1,2-sulfenyl... 

David and Lubineau191 reported the synthesis of pseudocytidine [5-/3-D-ribofuranosylcytosine (270)] and its a anomer by a procedure analogous to that used in preparing pseudouridine.155-157 Thus, 2,4 3,5-di-O-benzylidene-a/de/iydo-D-ribose (223) was condensed with the dilithio derivative of 2-0,4-N-(trimethylsilyl)cytosine, and the resulting, epimeric, acyclic derivatives were subjected to acid-catalyzed cyclization. The anomeric configuration of the free C-nucleosides was ascertained by spectroscopic methods and by their transformation into a- and /3-pseudouridine in the presence of nitrous acid. The anomeric 5-(/3-D-ribofuranosyl)isocytosines have also been prepared by Fox and coworkers.1913... 

The chemistry of acyl nitronates derived from secondary AN has received much more attention. Yoshikoshi and coworkers (226-228) developed a reliable procedure for the synthesis of these derivatives from readily available precursors (ketones and a-nitroalkenes), they demonstrated that the resulting acyl nitronates (123) are convenient reagents for the preparation of various heterocyclic and acyclic derivatives (226) (Scheme 3.104). 

The preparation of C-1 glycals has been largely addressed by synthetic modifications on cyclic carbohydrate derivatives, although strategies that rely on ring forming reactions from acyclic derivatives have recently emerged (Fig. 1). 

Further efforts in related series afforded structurally similar DPP-4 inhibitors such as acid 13 (E. Parmee, unpublished results), thiazole 14 [35] and cyclopentylglycine 15 (Figure 17.3) [36], Although 15 did not show improved selectivity over DPP-8/9,13 and 14 showed some improvement in selectivity over these counterscreens. Nevertheless, none of these compounds exhibited sufficient selectivity to merit further pursuit zwitterion 13 also possesses low oral bioavailability in rats (F < 1%). Consequently, focus in the a-amino acid series shifted to acyclic derivatives. 

Amino substituents in acyclic derivatives have been discussed by Eggert and Djerassi (396), who emphasize structural and conformational effects, whereas Batchelor has investigated SCS(NH2) and nitrogen protonation shifts in methylated cyclohexylamines (424). The, 3C NMR spectra of amino acids have been compared with those of amines and carboxylic acids (425,426). The transmission mechanisms of amino, ammonium, trimethylammonium, acetamido, and di-acetamido groups have been examined by Faure and co-workers (427), the SCSs of nitro groups by Ejchart (400), and those of azido functions in steroids by Lukacs and co-workers (428). 

For example, Nakamura and Kuwajima [15] have described 1-alkoxy-l-trimethylsilyloxycyclopropanes (15) -prepared by reductive silylation of alkoxy 3-chloropropanoates-, which react with aliphatic aldehydes, but not with ketones, in the presence of one equivalent of TiCl4 to give good yields of y-lactones 17 through the acyclic derivative ethyl 4-hydroxybutanoate (16) (Scheme 5.10). With aromatic aldehydes and their acetals the reaction leads directly to acyclic 1,4-D derivatives. 

Scheme 6.34. Leaving group and reagent dependence of allylic substitution in acyclic derivative 163.

In order to determine whether the partial suppression of the free-rotor effect was required for the success of the ODPM rearrangement process, the study was extended to the aldehyde 33 [51], When 33 is irradiated (15 min), under similar conditions to those used for 29, the cyclopropyl aldehyde 34, resulting from an ODPM rearrangement, was obtained, as the tran -diastereoisomer, in 90% isolated yield. This result demonstrated clearly that the ODPM reactivity of (3,y-unsaturated aldehydes is not restricted to cyclic compounds, such as 29, but can also be extended to acyclic derivatives. Therefore, the suppression of the free-rotor effect is not essential for the success of the rearrangement and the reaction is probably controlled by both the excitation of the molecule to the TiCtt, -it )... 

Alterobactin A is a cyclic mono-catechol-bis-hydroxycarboxylate bacterial siderophore with an extremely high affinity for iron(III) (see Table 12). On hydrolysis n the absence of iron) it gives an acyclic derivative which forms a bis-ligand iron(III) complex. ... 

HMF has been discovered for the first time in 1895 by Diill and Kiermeyer who independently introduced a method of synthesis of HMF that they named oxy-methylfurfurol [65, 66]. Later, Haworth and Jones studied the mechanism of this reaction and showed that the formation of HMF involved a triple dehydration of hexoses [67]. Other studies performed by Van Dam, Kuster and Antal showed that the dehydration of hexoses (especially fructose and glucose) involved two possible pathways (Scheme 5) [63, 68, 69]. The path 1 involves the dehydration of ring systems (fructopyranose or glucopyranose), while the path 2 is based on acyclic derivatives (glucose and fructose open chain). 

Achiral cycloalkyl quaternary amino acids have also been incorporated into model peptides using 5(47/)-oxazoIones as intermediates. A comparative study between cyclic and acyclic derivatives has been described. The influence of the ring size of a homologous series from l-aminocyclopropanecarboxylic acid to... 

Both enantiomers of 2-methylamino-l-phenylpropanol (ephedrine, 1), which are commercially available and relatively inexpensive, have been used as auxiliaries in many syntheses of chiral compounds. Ephedrine can be used for amide alkylations both directly1-3, or as derived heterocyclic compounds (see Sections 1.1.1.3.3.4.2.1. and 1.1.1.3.3.4.2.2.). Acyclic derivatives of ephedrine are discussed in this section. For example, either enantiomer of ephedrine gives A-acylephedrines 2 in good yield without epimerization if treated with an anhydride at 65 °C for 10 minutes2. 

Inasmuch as the factors leading to closure of a new oxygen-containing ring are strongly influenced by any ring already present, it is evident that the displacement of sulfonate groups in acyclic derivatives may follow a course quite different from that observed for cyclic derivatives. 

The atom X, still in an sp hybridization state of a heterocyclic system, is at one end of one unsaturated Y=X bond in a nonaromatic ring. The same conformational profile as in case 1 is expected if reference is made to vinyl ketones or a-diketones, which are corresponding acyclic derivatives. 

In the acyclic derivatives, the possibility of ortho-ester formation between C-l and C-2 would be lessened by a more facile reaction of the carbonyl group with ammonia. 

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