Introduction The Diels—Alder DA reaction developed into a powerful synthetic method since its discovery more than years ago [ 1 , 2 , 3 , 4 , 5 , 6 ]. As a member of the class of pericyclic reactions, this cycloaddition is particularly valuable due to its stereospecificity and the often observed high diastereoselectivity. This last feature is highly desirable and turned out to be the key to the successful synthesis of molecular belts [ 7 , 8 , 9 , 10 ].
In a series of classic papers, Stoddart et al. We follow their nomenclature and use the descriptors syn and anti for the relative configurations of the endoxide bridges and exo and endo for the relative configurations of the hydrogen atoms of the bisdiene after DA reaction. As a consequence of the pronounced diastereoselectivity of the DA reaction involving strained bisdiene 1a and bisdienophile 2, the reaction in ratio also yields a single diastereomer of the adduct 5a Scheme 1 c.
Remarkably, the reaction in ratio does not produce polymers since the reactivity of the second diene unit is reduced once the first diene unit has undergone a DA reaction, as discussed by Stoddart et al. Due to the diminished reactivity, the reaction with another bisdienophile building block requires high pressure Scheme 1 c.
In this way, it was possible to synthesise molecular belt 7a, a potential precursor of [ 12 ]cyclacene 8 Scheme 1 c. First envisioned by Heilbronner [ 16 ], fully conjugated molecular belts consisting solely of linearly fused benzene rings, cyclacenes [n]CA, where n is the number of benzene rings , have not yet been synthesised although related systems have been achieved, for example, by Gleiter et al.
More recent attempts by on-surface synthesis using a tetraepoxy precursor further demonstrated the challenge in achieving full conjugation [ 10 ]. Synthesis of conjugated molecular belts and structurally related molecules has been reviewed in the literature [ 22 , 23 , 24 , 25 , 26 ]. The resistance of cyclacene formation also under the conditions of on-surface synthesis is remarkable as the combination of ultrahigh vacuum, submonolayer coverage, and catalytically active single crystal transition metal surface is employed.
These conditions previously allowed the production of acenes [ 30 , 31 , 32 , 33 , 34 ], the highly reactive linear analogues of cyclacenes [ 35 ]. These were installed synthetically by DA reactions involving bisdiene 1b that is closely related to 1a employed by Stoddart.
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Rain is rare with 2 to 4 days of significant precipitation per month. Fall is the second busiest for tourism, which makes it a good time for those looking for things to do. Winter June through August The middle-year months have very comfortable weather with high temperatures that are comfortable. These months see moderate precipitation with about 0 days of precipitation per month. June — August is the busiest season for tourism in Isabela Island, so lodging and other accommodations may cost more than usual.
Spring September through November Spring daily highs range from It rains or snows a trivial amount: approximately 0 days per month. Tourism is the slowest during these months due to the weather, so hotels may be affordably priced. Summer December through February Weather is perfect this time of year in Isabela Island to be enjoyable for warm weather travelers. The average high during this season is between On average, it rains or snows a smalll amount: 0 to 5 times per month.
Rain is rare with 2 to 4 days of significant precipitation per month. Fall is the second busiest for tourism, which makes it a good time for those looking for things to do. Winter June through August The middle-year months have very comfortable weather with high temperatures that are comfortable. These months see moderate precipitation with about 0 days of precipitation per month.
June — August is the busiest season for tourism in Isabela Island, so lodging and other accommodations may cost more than usual. Spring September through November Spring daily highs range from It rains or snows a trivial amount: approximately 0 days per month. Tourism is the slowest during these months due to the weather, so hotels may be affordably priced. Summer December through February Weather is perfect this time of year in Isabela Island to be enjoyable for warm weather travelers.
The average high during this season is between On average, it rains or snows a smalll amount: 0 to 5 times per month.
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Elizabeth Holm - Machine Learning and Making Things; House Manufacturing Caucus
